When it comes to ventilation... less is NOT more Happy
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.