Actually, the proper rule is to put amps on the bottom if they are passively cooled as most are as this will put them closest to the coolest air.
Very realistically, there should be an air gap between all gear in any rack. I don't care what the heck people use, but now that film canisters are obsolete, come up with something! Get a piece of wood, paint it to match, and get it between the receiver and the component. Then put another spacer between each component.
The 1/8" rubber feet on the bottom of the different components is not enough to meet the proper specs. I would shoot for about a 1" minimum air gap to promote air flow around the gear.
In a fully open rack (front, sides, back open), a half inch air gap around any and all gear may be sufficient, but I would stick with 1".
This is the difference between a good A/V rack and a general consumer one. The consumer ones look good, but typically don't have enough depth and don't often have enough shelves in them to handle all the different pieces of gear and don't often have adjustable heigh shelving in them to deal with the different sized pieces of gear.