1. Why in most pictures do I see the audio equipment behind the speakers or mostly on the floor??
Between the speakers, where the center image is, you really want to keep that area open and free if possible with the exception of diffusion or other acoustic treatment. The vast majority of musical information is in this area and having a "bunch of stuff", bookshelf, racks, etc will merely get in the way. Keeping in mind, depending on the design of the rack and circumstance, its entirely possible it could helping diffuse which is good but thats a crap shoot. Bottom line: If you can keep the area in the middle, center image free from stuff, this is probably going to yield the best results.
I recommend diffusion if its an option.