Usually I find that wall warts falling off is due to the receptacle being "past it's prime" and replacing it takes care of it. I can't think of when I've seen a polarized wall wart, so you can sell place one in the lower outlet facing down, and the upper one has to face up usually because they hang down and interfere with the lower port.

Aside: One regret I have in my kitchen remodel is not replacing all the boxes with dual gang.