Sewing up a mare is done if the tilt of thier twat is off. It can catch tiny particales of poop and cause infections. That was my understanding of the procedure. There may be more to it then that.
I had my mare sewed on a vets advise before I bred her the first time. It's a pretty gross procedure closing and opening. I have since come to the conclusion that it is done a bit to often on mares. That's why I didn't have her sewed back after Dot. I would say wait till the mare has her first problem with an infection before doing anything. She's never had a problem. Also my understanding is it usually doesn't have to be done till after a mare has had a foal.
Becky
You said it a lot more artfully than I did!