When it comes to ventilation... less is NOT more
It also depends what devices you have... computers & projectors have fans that blow to circulate air through them, but a lot of home A/V equipment ventilation works off of the principle that warm air rises & they have vents on the bottom & top of the box. If this is the case... I'd be careful about doing any stacking at all. I don't have anything above/below my AV receiver. If your equipment is cheap, I wouldn't worry about it too much anyways, but a few INCHES (not centimeters) would be good.
We just recently got a new home theatre system (Panasonic SC-HT05) and it works great. The entertainment armoir we have only has two spaces for VCRs, DVD players, or other things. In those two spaces are a DVD player and a digital cable box. I decided to put the receiver under the cable box because there was enough room to fit both. Above the cable box are only a few centimeters of space. I'm wondering whether or not this could cause any problems. Any feedback would be greatly appreciated.