Good manners says to say thank you when somebody answers your question. E-mail load control suggests not replying "thanks" because it's too much e-mail and people would rather have less. I'm not sure what StackOverflow expects. I got an e-mail notification today "3 Questions Have 1 Answer - Stack Overflow". When I clicked, only 1 of the answers was new to me. That one was a short, correct, and complete answer. :-)
I cannot vote it up because I don't have reputation.
Should I comment "thanks" on it?