The whole discussion of subwoofers (are they necessary, are they overkill, do I need them) is going to be subjective. Whether you need them or not is clearly a choice.

But excellent question... the disclaimer is a case can be made for either yea/nay....

In general, 5” and 6.5” (7) have a matching Subwoofer in the Yamaha MSP (Monitor Speaker Professional) lineup.
In the HS (a Home Studio) lineup, the 5”, 6.5” (7), and the 8” have a matching Subwoofer.

Monitor Speakers are sold separately — in critical work situations studios configure different mix suites for different uses. If you are doing music for movies you might be working in any number of different surround sound situation 2.1 (simplest L/R + sub) through to 12.1 where you are trying to place the audience inside the environment.

Do you need a subwoofer... the answer is it depends on what your purpose in music production - the target audience. If you go with 5” stereo, you will appreciate the subwoofer just for the fullness it brings overall. The 8” stereo can usually carry fullness without a need for a subwoofer. The model in the middle usually called a 7 (but is actually 6.5”) it’s your call...

Back in the day, we would do these two track mixes in the studio, then to get an idea of what this would sound like on the dance floor, we’d take the mix up to Studio 54 (my partner knew one of the DJs) we would crank it through the system in the middle of the afternoon - stand in the middle of the dance floor and listen. We didn’t have Subwoofers in the studio and it took several trips before we felt comfortable that we could recreate ... but what it teaches you, is that you have to mix for your audience.

If you are guessing about what is going on in the low end of your mixes, you’ll keep getting surprised when it sounds good here, but not through the car’s sound system, or it sounds good in headphones but no where else... you have to get monitors that give you the truth.

“By the time you hear your subwoofer - you almost have to much” — is the rule of thumb for subwoofer. When they are doing their job properly, you are unaware of them. You just know the music sounds balanced for the space you’re in.

If you are mixing for games, or for the dance floor, your audience will appreciate it, if you have clear focus and can dial in the low end properly.
If you’re doing other types of recording, you may not necessarily use the subwoofer in the same way... you may not require it.

Rise above Consumer Hype:
Do not fall for the consumer hype — most home computer sound systems include a subwoofer and two small “satellite” (really means: two horrible little tweeters) speakers. The subwoofer moves a bunch of air which is supposed to be impressive, and the little guys give the sound some clarity... but on a scale of 1-10, overall it is typically a solid “2”... yes, this is subjective and I’m painting a broad stroke about included computer sound, but let’s just say they include a sound system (and they can check the box)... but you do not want to use it to critically listen to music.

Studio Monitors are designed for listening to detail...
In consumer world the subwoofer is what impresses the untrained ear. When you hear how different people listen to music in their cars, you understand that for some it’s all about the bass... but as a person doing the mixing of music — it’s all about all the frequency ranges, and getting each to do what it should.

Knowing this - make sure you are getting the subwoofer for the right reasons... (your audience is low end oriented, you’re doing sound for film, for gaming, for dance floor, etc)...
If you’re getting it just to playback (as in casual listening) you may find the Studio Monitor’s “matched subwoofers” oddly expensive compared to consumer flufferwoofers (if you know what I mean). I’m being nice, or trying to be nice.

Final Word: If your Home Studio is upstairs from, or next door to neighbors where you share walls/floors/ceilings etc... remember your neighbors! The wavelength of bass notes are measured in feet. (It can be 50 or more feet). They will hear, feel, and suffer with your low end. When low frequencies encounter a surface, instead of bouncing back into the space, they hit the surface and spread out - putting that surface in motion. Just FYI.

My main concern regarding a subwoofer is the pianos - CFX, Bosendorfer...etc...

A grand piano goes down to 27 Hz - so if you are after a "real" presentation of a grand piano - you would "want" a subwoofer - correct?

I guess what I was asking is - if I am using a subwoofer to get to the lower frequencies - do I "need" the 8" monitors - or would it be "better" to get the 5" or 6"...?

Very good question!!!

This comes down to the make and model's gap between the capabilities and sweet spot of the driver and the tweeter, and the size of the sub.

And the crossover sweet spot of the sub versus the drivers.

Most 8's have a big gap between their sweet spot and the tweeter, meaning the tweeter is struggling low and the 8 inch is struggling high, to get that crossover area to be great, largely because they use the same high tweeter across their 4's and 5's and 6's of their ranges as they do in their 8's. Those that don't, often have a tweeter that's not quite as good up high to get the blend right.

Further, this spot where both the 8' and the tweeter are struggling to crossover nicely is where much of the nuance in a piano exists, that defines it as a good and listenable piano... particularly the harmonics in here.

Much of what's great about EDM (particularly Trance arpeggios) is brought to life by what's in this spot, too.

And just about all that lovely sparkle of FM sounds is in this space, too.

At this point of realisation, things like the triple speaker approach of things like the KRK 10's (staying within sane budgets) become really attractive, because you can cover almost all you want to do with these, and they do almost all of it well.

https://www.sweetwater.com/store/detail/Rokit103G4--krk-rokit-10-3-g4-10-inch-3-way-powered-studio-monitor

A pair of these is about the cost equivalent of a pair of 5's and a sub and (I think) a better alternative to the path of a sub and fiddling with crossover and positionging for something as focused as working with a synth right in front of you.

There's also a subwoofer body suit you can wear apparently. As I saw advertised at Believe In Music. For rattling your bones according to the marketing information.

https://subpac.com

Just adding a personal anecdote... I bought my JBL LSR-305 monitors back in 2014 but decided to skip the sub. These monitors have served me well for all those years. I never felt there was a lack of bass. However this past December, Amazon had the matching sub for 1/3 off, so I picked one up. I have to say it makes a huge difference. Granted, I'm still at the stage where I'm enjoying the floor rattling base. I haven't tuned it to make a fuller sound as Bad Mister describes above. I was worried I had made a silly purchase but I'm really enjoying the extra dimension to my sound.