Try checking the other threads here:http://www.avsforum.com/avs-vb/showthread.php?t=330436http://www.avsforum.com/avs-vb/showthread.php?t=841019http://www.avsforum.com/avs-vb/showthread.php?t=987802
Specifically though (and let me preference this by saying I am a newby and don't have platform/riser experience), I would think that two BK LFEs in each three chair platform should do the trick - three if you want to be aggressive. I don't know if it is best to have one under each chair or to have them as close to the middle as possible to allow for maximum flex. I believe in general it is best to have them as far from rigid sides as possible. For instance if I was going to mount some under my floor, I would (if I could) mount them in the middle of a joist span, where the BK could force the joist to flex/vibrate as much as possible.
No, BKs or any other tactile device is not "important" per se, but they are REALLY cool and fun and add an whole 'nother dimension to movies and music. What you feel will depend on how you set the crossover filters, but in general you will feel movie and music content in the 1 Hz (I've heard 0 hertz mentioned, but I don't see how that is possible, 0.4 hertz sure) to say 100 to 120 Hz range ), based on the seemingly favored 80Hz roll-off. What that means is that explosions and crashes will shake you (and perhaps the house), you will get jarred by gun shots, light sabers and fire will make your body hum/vibrate, when the rhino beast hits the column in SW2 you will bounce, you will feel waves crash into your chair, in music you will feel the drums and bass guitar, I could babble on, but hopefully this gives you the idea.