I make it a habit to answer all of my PMs. If I forget, then I feel really bad. I hate hate hate it when I PM someone a question or give them a compliment and they don't PM me back. It tells me alot when they don't care to give a "thank you" for a nice compliment. However, if I see a PM that doesn't need to be answered, like the end of a conversation, then there's no need to answer it and the same thing for people I PM. That doesn't bother me at all.