That question also depends on which German cities you are talking about.
If you are talking about Dresden, then no,the bombing was not justified.
There was no military reason for it at all.
If you mean Berlin or any of the cities in the Ruhr Valley, then yes there was military justification for it.
Many of those cities were part of the German industrial complex, and as such destroying them was a military objective.
Berlin, as capitol of Germany, was an important city, and destroying it was a valid military target.