I would do them whenever I'd think about it, but never did them regularly or anything. I still had a very good labor/delivery, but maybe it would have been even better if I did them more!
BTW, the best way to move those muscles is to think of trying to stop the flow of urine.
One MORE thing! I remember from my pregnancy yoga video, that you "climb the floors of a building." Level 1 is just barely tightening the muscles, then level 2 a little bit more squeezing, and so on until you get to 5. Then you work your way back down. 5, 4, 3,2,1