You need at least an inch (preferably more) of airspace. Even with that you could get significant heat buildup (I actually drilled a grid of holes in the back of my entertainment center to help ventilate it more and the receiver still gets toasty.
I'd look for a slimmer line receiver or think of a way to get your components into a different piece of furniture.
I have an alcove in my living room with shelves where my stereo components are kept but the space between shelves is only 5.2 inches tall. The new receiver I want to buy is 5.9 inches (a standard 100watt model but highly rated) If I take the little feet/pads off the bottom of the receiver it will fit. But I'm wondering if it would be too tight or enclosed or may overheat or something? (both the front and back of the shelf is open)
Thanks for any help.