W/ DS1 the MW even had to move my cervix forward a bit to do it (ouch), but I was still in labour by 8 p.m. that evening. I was exactly 40 wks. W/ DS2 I was 39w5d and had one, was already 2 cm and 50%, contractions that night that were on/off for a day or two, then a v. quick L&D right on my due date. I'm thinking I will have one this time, too, but am also waiting to closer to 40 wks (I think my appt is 39w3d?). FWIW I also was using EPO orally starting at 36w, w/ full dose reached by 37w w/ an application vaginally at night.
HTH. It truly is a personal decision. As a PP said, it is an intervention of sorts, and so not w/o risks. What tipped me into it was a) an obsessive desire not to be overdue in the summer where I am...ugh. and b) that my pgs so far are rather normal, w/o sig. intervention anyway other than this.