While there are benefits to this there are also risks. Make sure you get someone who knows what they are doing. If it hurts, they are doing it wrong. Also make sure they use gloves and finger condoms. You do not want a girl sticking her finger in there when it has been in someone else's ass as well. And be careful of sharp nails.
Back in the SP days I used to see this Korean girl named Yuki, aka Sara. She would rim you then give you a prostate massage while stroking and sucking your cock. It was the most explosive finish I'd experienced. That was always round 1. Round 2 consisted of me fucking her senseless and her moaning loud enough for the neighbours to hear. She could be a little crazy after a few drinks but she was fun. Miss her. But I digress.
It should be a massage not an ass fucking.