said by nunya:that is exactly the advice my hvac contractor gave me, never cover the whole thing - not to use one of those full-cover things, because it traps moisture and just encourages the unit to corrode. He told me to put a square of plywood on top with a brick if I wanted to keep the leaves out.
I have a piece of marine plywood cut to the shape of the A/C top. It keeps the leaves out. The sides are still open so air can move freely and dry out any water that does accumulate inside.
A cover is not unnecessary, I just hate cleaning the leaves out each year.