Looking for some direction around designing cabinets for a new home theatre

1) I'd leave a minimum of an inch on each side and a minimum of two inches above, especially for the receiver.
2) Anything you can do for ventilation is good. Some people use a cooling device that's meant for laptops, to cool the receiver.
3) The ideal height for a tv is for the center of the screen to be at eye level, when sitting down. That's why tv stands are built the height they are. Mounted high on a wall (like above a fireplace) is not the best place. It has a cool factor about it, because people saw a decorator on HGTV mount one that way, and they say, "oh, cool. I think I'll do that."
4) Nothing I can think of at the moment.

I have thought for years, that I'd eventually build a bookshelf/component system, that takes up a whole wall. The difference of mine being, I'd build it about two feet out from the wall, but braced to the wall, so it's sturdy, and put a door at the end of it (do it so others wouldn't know it's a door), so I'd have room behind it to move around. Then when I need to undo cables, or connect cables, it would be easy. I'd also make sure an AC vent is behind it, to keep everything cool and well ventilated. I've not seen this done before, but thought it was a good idea. I worked in a cabinet shop for eight years, so I wouldn't have any trouble doing it. The reason I haven't is because we don't own the place we're in, and I don't have a workshop. So, it's looking like it may never get done, for me anyway. You're welcome to use the idea, if you want to go to that extreme with it. Could look very impressive in walnut, cherry, or even oak, but wouldn't be cheap.

You might also see some ideas you like here: www.diynetwork.com/search/results.do
Good luck!