Definitely not too often, and it's always voluntary. It's actually illegal to force nurses to stay over their shift in Massachusetts unless there's a major emergency. (Thank god)
Yesterday was a weird situation where at the end of my shift, there was a peritoneal dialysis patient that needed an out of the ordinary treatment. And on the team of dialysis nurses I work per diem with, I'm the one with the most experience with peritoneal dialysis. It would have been way more difficult for one of the evening nurses to do it, so I agreed to stay (and they gave me a bonus to stay, so even better!)