An easy rule of thumb for people people who don't know their way around acoustics well, is that "smaller rooms"(home audio applications) will likely do better with more diffusion, while larger rooms will require a bit more absorbtion to balance out the reverberation equation. Smaller rooms(for HT/music purposes...but don't forget HT systems can get away with more absorbtion in mid/treble) of standard drywall/stud construction will absorb less bass, while larager rooms absorb much more bass thoughout the spectrum. If you don't absorb a little more of the midrange/upper frequencies in a large room(and say, have mostly hard floors and flat walls), the spectral tilt and reverb of the room will seem "too bright" and "top heavy"! It's the converse in a smaller room. You have a shorter reverb time in a smaller room in the mid/treble. So coupled with the fact that there's "excess" bass reverb and less bass absorbtion in smaller rooms, you need to keep the reverb time higher in the mid/uppers.
That will all balance out in terms of how(and with what materials) you treat your reflection points in the room as well.
Case in point: In a smaller, say 12x14x8 foot carpeted floor room, where you are doing a balance of HT/music, you should consider perhaps doing reflection/diffusion treatments on the side and back walls(covering first reflection points), with maybe diffusion up front as well, but possibly treating the first reflection pionts on the front wall(front speakers) with absorbtion...still maintaining some diffusion upfront. I think you'll find this will still have "life" enough in the sound for music, but be more than acceptable to handle HT as well.
For a larger room(domestic house rooms of course), say 19x24x9, you might consider doing more absorbtion around the front and front/sidewalls for acoustic treatment in various places, but maintaining diffusive/reflective treatment(or elements) along the back/sides and back of the room...treating front reflection spots with more absorbtion than in a small room. Of course, you'll need to consider the overall reverb and balance of the room, and you're ear will be the final determining factor. Considering a "medium sized room", you'll have to balance somewhere "in the middle" of the above two scenarios.
Still, ofcourse, there's a lot more to it. But I think if you experiment, you'll find what I mentioned about right for most peoples taste.
F.Alton Everest's "master handbook of Acoustics" states(along with a bunch of other acoustic publishings) that "many professional acoustic engineers will even dissagree as to what exactly is the best overall reverberation times, and approaches to treating listening rooms", and that "tastes differ!"....it's like anything else really. But, experiement around with the stuff if you will, and you'll get an idea yourself.
One more word to consider here, and that's that different speaker systems do diffent in differnt acoustical environment perceptually! For instance, Horn speakers, some THX designs, bipols/dipols and say planar or electrostatic speakers will possbily require different room acoustic treatment approaches! where there's more "dirrect" radiated sound patterns you'll "less of the room reflections early" But where there's more "back wave" sound distibution, you might find you need more "reflection and diffusion" around the room to give a proper sound from that kind of speaker system. So there's no "one best" room acoustic treatment approach in all pertinent aspects...YOU GOTTA EXPERIMENT IN YOUR ROOM WITH YOUR GEAR!...and for your own tastes.(hint, no two recording studio's are alike either!) For that matter no two audio systems are usually alike, and there's an infinite varriety of domestic house rooms out there as well.
You can check out some helful acoustic websites(which you'll have to dig up yourself...but they're there) to figure out room modes, reverberation RT60 times, even plot potential room acoustics treatment approaches.
Still, there's no replacment for the "professional" when it comes to "doing this acoustics stuff right!" If you need it done right the first time, you can always call the professionals! Rivesaudio.com is one to consult if you need someone to contact online for acoustics help.
Otherwise, have fun and good luck with your endevore
Hope this helps