All are viable, since they will all bring some form of support to the group too.
We had all sort of specs in our guild and we were half way into clearing AQ40 before TBC hit.
Keep in mind, our raiding started a good year into Vanilla. Getting 40 people was hard enough, couldn't be too picky.
Min-maxing was not such a big deal since bosses did not have a hard enrage timer.
I have no experience in Naxx, but I could imagine it would have been more demanding.
TL;DR Play what you want, you'll clear AQ40. I speak from experience.