I've had one baby born the day of a S&S and another the day after. I was already post dates (8 & 5 respectively) and 2-3cm dilated so it was possibly ready to start any time anyway.

I did have one at about 39.5 weeks (can't remember exactly) at 1cm. It was VERY painful and I had lots of cramping and show that night but didn't do anything as I had to go back the next week. I wished I'd not requested it as it was my choice to try it.