Yes, definitely wait until final grades are posted. Otherwise, some people may get the wrong idea.
I read an article on the Reader's Digest site once: "Things Your Child's Teacher Won't Tell You" (obviously, this is K-12). One of the things they said was: when it comes to holiday gifts, GIFT CARDS are appreciated much more than things like picture frames, etc. College professors would probably feel the same way.
Just a thought. The dollar amount doesn't have to be very high (no more than $20, I'd say). If possible, make it one of those "use-anywhere" Visa gift cards--that way, they're not being "locked" into a particular store.
If that's not possible, a bookstore gift card is almost always appreciated by everyone. I love those. 
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The existence of the leader who is wise
is barely known to those he leads.
He acts without unnecessary speech,
so that the people say,
'It happened of its own accord.' -Tao Te Ching, Verse 17