I had previa earlier in this pregnancy. By 20 weeks the placenta was flush with the os and my homebirth midwife said she would be comfortable delivering with that. She said she's had other mothers who've had no problem with it. I had another ultrasound at 28 weeks and they said it was just fine because the placenta was "a couple" of centimeters away. I didn't ask how many a couple was, though I probably should have. They assumed I was delivering at their hospital and apparently they considered the problem resolved because they didn't ask for another ultrasound.
Another thing to keep in mind is that the uterus grows the most in the third trimester and thus the placenta has the most chance of moving then. So try not to stress about it (I know that's not easy to do).
On a side note, this baby is the lowest of all four of my pregnancies. Right now at 36 weeks, her head is completely engaged and has been for a week or so. I don't know if that has to do with placental location, but it's an interesting correlation.