Greg Pierce (of Drafts for iOS fame):
If your app is free, and they follow your link, then buy something totally different within 24 hours, you win.
Some people will mindlessly bitch about iTunes affiliate links as if Apple would otherwise have no idea what you browsed while in the iTunes or App Store. If you post iTunes links for things you haven't actually used then, yes, that's gross. If you genuinely like something then it's awesome that you get this kind of commission for simply pointing people to it. Win-win.