Mitty

If it looks like it has stretched and no longer laying flat, then yes, I would re-baste, but otherwise I would just keep going, checking occasionally to make sure everything's still looking good.

In general I haven't had problems with the fabric bunching/stretching as long as everything is flat and smooth in the beginning Except when using flannel. With flannel, even when I've had in tons of pins it sometimes bunched up at the seams. I unstitched (which was a huge pain because it was ditched) and redid the seams that had bunched, and then I went really slowly and held the fabric taut any time I was at an "intersection."

You might consider using Elmer's washable (repeat: washable) school glue if you do need to rebaste. Just use tiny, tiny drops, but you can have many more tiny drops of glue than you would ever have pins, so the whole thing is more secure.