True that, but don't be too disappointed if you don't get a reply back. Otherwise, there should be no problem PMing old members. If they never log in again, they will never see it, and if they do, they're no longer inactive
Some people reply to threads instead of PMing the original poster, though. In the case of the original poster being MIA, that can be a tad annoying, as it bumps the thread, bringing it to the attention of other people while it's outdated.