As Japan has been extremely obstinate in even admitting to its numerous warcrimes of WW2, I don't think they deserve to have an appology for USA and Allied attrocities (much much much fewer than Japan's). If they'll fess up and admit to their own wrongdoing, it's only fair that the WW2 Allies do the same. Because Japan is so stubborn, the Allies will likely never have to worry about it. Japan asking for an apology would be hypocrisy.
Germany, on the other hand, has been very forthcoming and has offiicially appologized for virtually all of its attrocities -- and there are a lot of them. I think that giving them a "We're sorry too, Dresden was very bad of us." can't hurt, can it? Its one small case where the Allies committed itself to slaughtering non-combatant civilians enmasse and unnecessarily. Even if you buy into the Allies official versions of things and Dresden was a massive producer of military hardware for Germany ... why not take the opportunity to better relations with Germany anyway?
Mostly it would be conditional on whether Germany has made an official apology for the deliberate targetting of civilians with V1's, V2's and the London Blitz. If Britain and the USA do not consider any of their actions in the war to have been criminal, then the London Blitz and all the rest was 100% fine and just an acceptable part of the war.