My first baby was posterior, she turned sometime during labour, I'm not 100% sure when but I think it was in the last couple of hours. I had more back pain in labour with her and labour was very slow, I ended up with the drip and an ARM to speed things up. She also wouldn't drop low enough until the last few hours of labour.
My second was also posterior until right before he was born. Again I had some back pain, his head was too high and although labour progresses a lot faster this time I was stuck at 9 and then 10cms for a while. We also had a shoulder dystocia which is probably unrelated but I do wonder if it was partly because he wasn't in the ideal position still.