To each his own, but not for us. While in transit, we've spent a few quick overnighters at rest areas, and once at Cabelas. Without prior permission, I would be sleeping with one eye open, and I don't care for that.
Having said that, I've never been in the position of it being late, and not having anywhere to overnight. If I was forced, I might park at a grocery store, and make sure to patronize them in the morning. That way, I would be in compliance with the signs that say "for customers only". You can disagree with me if you want to, but my opinion is that you shouldn't park on private property without permission.