I just had my 35 week appointment with my midwife and I asked her when she usually starts doing vaginal exams because I was expecting to have one next week or so. She said, "When you're in labor."
So we had a long discussion about it. She sees no benefit in knowing dilation or effacement before labor begins, because it does not really mean anything. You can walk around at 3cm for weeks or you can be closed up tight and go into labor the next day.
She would use one to check for vertex position if she could not easily palpitate the position, but would not go near the cervix unless I request it. She followed this with, "But you can check yourself if you want to." And she described to me what to look for, which was great!
As far as checks during labor go she likes to do one as soon as she comes (for the home birth), to know what the starting point is. She recommends the before pushing check for first time moms, but if you labor the baby down gently without forced pushing (some women feel the need to do this and some don't) there is no need to check.
So, that's my MW's opinion.
As for me, I like knowing things. I check my blood sugar for the fun of it, if I had a sphygmomanometer I'd check my BP for fun too. I'm too much of a scientist... I'd be getting checked every 20 minutes and making graphs during labor if it were practical.
I'll probably request more exams than she usually does because the curiosity will be killing me.
