I haven't read all the response so this may have already been said. Just a gimmick for the movies to show that the guy is actually cumming. Called the "money shot". I don't do it unless she asks for it. I always ask ahead or at least let her know I'm ready and let her say where. Just the polite thing to do.
Where ever she wants it. I prefer in her mouth but it is up to her. I would never cum in her pussy without permission.