This is a very touchy subject. I have had some very different views on the situation from some respectabely skilled people. Some insist on atleast 50% response rates, other like myself aren't worried if people respond at all, and some people insist that it is completely unnecessary and people shouldn't respond unless it is to give advice on the list quality. So for that I think a mid point should be drawn where people are thanked and also supported to improve their lists.