Not to highjack the thread, but I've observed many fine orchestras and bassists, and the highest frequency of misuse of the spinal column is among players who sit. For example, in drurb's link to "apparent amateurs", Joe DiBello. The other three aren't playing, and can't be judged either way.
My own teacher, Linda McKnight, (Manhattan School of Music) is adamant about standing to play.
In fairness to someone whose posts have annoyed me, he is, in fact, deferring to someone with more proven knowledge and experience, a quality the absence of which drove everybody nuts on other threads.