Speaking as a teacher, I think any token of appreciation is great! Last year I got some chocolates from one student for Christmas, but just cards from other students. The classic card was one that read, "You're not my favourite teacher but have a good Christmas anyway." At least it made me laugh!
This year I have a year 7 form group and got Christmas cards from most, if not all, of them and I got them all Christmas cards as well. The cards were all very sweet and very much appreciated. As someone else has said, teachers don't often get thanked, and we do work very hard, so any token - gift or not, is very welcome. I certainly don't think gifts are necessary, but I definitely appreciate the cards. I suppose I would feel a bit awkward if someone had obviously spent a lot of money on a gift, but I wouldn't worry about giving teachers some chocolates or other similar gift.