Solicited or not, I don't respond 100% of the time - you can end up with to-ing and fro-ing for no real reason other than both posters attempting to sign off as politely as possible.
Some posts quite obviously require a response - and I respond to them.
I would respond to any PM at least once - providing it's not abusive. Some posts do not seem to require a response or else I sometimes feel that there is nothing more to say on the subject and therefore I don't respond to them. Maybe sometimes I make the wrong call on this though and may inadvertently appear rude.
Unless the PM is rude/disturbing or in some way inappropriate I think it's bad form not to respond - after all, if you would prefer not to receive PMs you can always change your personal settings accordingly.