🙂 No, you're NOT an idiot, but you're also not going to buy this person a new phone if they install applications and find that, due to some insane set of circumstances involving goat cheese, a 16 ton anvil and Alan Thicke that they're unable to reset their iPhone to factory settings before taking it in and, as a result, are told their iPhone isn't covered.
Warranty service seemed to be
very important to the original poster and, let's assume, as you said, that no one really knows if it voids your warranty or not. We
do know that NOT installing applications will most assuredly not void your warranty. Along those same lines, NOT smashing it with a hammer will not void your warranty, NOT feeding it to the Bugblatter Beast of Traal will not void your warranty and NOT doing anything that voids your warranty will not void your warranty. 😀