Well the best time to prune maples is in the winter while they are dormant. Do not prune while in full growth mode. Wait till Feb or March before new growth begins. I'm not sure what you mean by topping. If you mean, trimming out the central leader, I do not recommend that. Depending on what species you have, your tree is probably what comes naturally to it--growing straight up and then widening with age. All topping will do is to make the tree unsightly and weaken it. Eventually another leader will take the place of the pruned one anyway. As far as your lopsided tree goes, there's probably a reason why the tree has grown this way. Is the one side more in the shade? That would be my guess. Anyway, if this tree is in the sun, it will even out eventually on it's own. Trimming one side, won't promote growth on the other. With trees, patience usually is the best practice because they right themselves eventually.