If you're talking about steel rims, feel free, hammer away. You're probably not going to fix a flat spot in the rim with a rubber mallet though.
If the rims in question are the cast aluminum ones in your avatar, the answer is NO. The cast aluminum will probably break when you try and hammer it back in place. If by some chance it doesn't, it will still probably break later, at a more inconvenient time. Cast aluminum really can't be worked like that. If its a cast aluminum rim with a flat spot and a bend, its scrap.