I had posterior placentas with DS1 and DS2 and they were both anterior when trying to labour.
With DD this year I had an anterior placenta and she tended to like facing it, but was in a good position by 40 weeks. The u/s confirmed that it was anterior, but I could definately tell the difference, as I couldn't feel the kicks as strongly as the prior pregnancies. They were definately felt later and more muffled.

A chiro that I saw worked wonders in getting the pelvis alligned and helping the baby to get into a good position as I was worried about the anterior placenta this time. I was also worried that it would be too low (I was trying vba2c and ended up with another c.s) and wasn't sure if there would be complications with the incision site etc. Turned out that the placenta moved up enough anyway.