I think the rule for me is if I see a person often enough where it would be faster to hand them a thank you note the next time i see them rather than send it in the mail, I don't write it. Birthday parties with my housemates, family etc.

But if I stay at someone's house for a night or more, am invited to dinner while I'm in town, etc., I like to send a note!

That being said, I think it's one of those things that everyone likes to receive, but never notices if they never received one.