When you are poking back up from the bottom just make sure you are as straight as possible from your previous stitch (until you are rounding edges, of course). I think that's about the best you can hope for in this situation. And lets face it, you're going to be looking at the top side of the quilt, right? Not the back side on a regular basis??? And even though you are using poly batts, you will probably still get a tiny bit of shrinkage after it's washed and absolutely no one will be able to see if your circles are completely round on the back!