There is a difference. The materials used have changed. Up to, say, 75% of the rated output ( number pulled from ass, wiped clean and given a pep talk ), they'll sound the same. More or less. It's when clipping and compression come into play that subtle differences can be heard by the user, not neccessarily anyone else.
I agree with this comment.
Personally I don't hear (or feel) much (if any) difference between valves until they are distorting/compressing. So unless I was really driving the output stage then I wouldn't usually hear any significant difference in output valves.
.. and because I almost always push the front end with boosts then it's not something I worry about. I do play around with pre-amp valves a little though.