I think the risk of C/S is because when they are coming out posterior, they can't tuck their heads in so that the smallest part is coming out first. So they can get a bit stuck. Maybe Kelly can come in here & explain it better.
Glenn did get stuck on his way out, but we managed to get him out with out any need for help (forceps etc). We didn't know that he was posterior till he was crowning. But When I described my labour to midwife friend she picked it striaght off with out being told that he was. Wish I had her for my labour, maybe we could have had it all done a bit quicker if the midwife knew what she was dealing with.