Definitely make sure the power amp has room above it to breath. I believe that the manual recommends at least two to three inches above the amp. Also, leaving the back off the rack simplifies wiring and improves ventilation if you can do it. Other components will also benefit from a bit of space above them (the Model 950, cable box, and possibly TiVo).

I have my amp at the bottom of the rack. There are a couple of benefits to this: it makes it easier to keep the speaker cables separated from interconnects (thereby reducing the chance of picking up noise in interconnects from the higher level signals in the speaker cables), and it gives the amp a cooler environment (heat rises, and heat from components below would lead to warmer air entering the amp). The power conditioner is not a significant heat source (at least mine isn't), but I try to arrange it so that the power cords can be isolated from interconnects easily.