Originally Posted by neuron

Actually, even though the fingerprint is on file, they can fingerprint you every time you enter.

I have been fingerprinted coming into the US (for trips originating outside NA), where I work. Since the documentation system has been going, I would say 80% of the time they will ask for fingerprints, despite them having my fingerprints on file, through Nexus, as well as another trip a long time ago.

As for going back and forth from Canada --> US, there is no documentation of when you leave, as even if you fill out an I-94, you would not surrender it for transborder travel.