I usually get a CBP re-entry stamp without asking. When I don't get a stamp, then I ask them. Now that I use GOES, you don't get a stamp at the kiosks...however it is a small price to pay for getting through U.S. Customs/Immigration upon return in a fraction of the time it would take you if you stood in the regular line.

I've had a few foreign immigration/customs agents deny me an exit stamp. They get annoyed if you ask them again and then hastily wave you through telling you that you're holding up the line.