Found if I don't start the second side right at the top it will leave a little hole. So, right at the top for me. Hint: your fold should be just a tiny smidge down from the raw edge. Worked wonderfully for me!

I stop slightly less than 1/4" or the width of the seam allowance. Perhaps a couple stitches and then back stitch a couple stitches. This allows the little bit of space for the fold just like the scant 1/4" in other seam allowances. When I start the second side I put need just into the folds. This holds the folds together and prevents bunching up that the pressure foot often does when starting at multiple layers.

It is easier for me to start at the very top.

I stop 1/4 inch from the corner, needle down and turn quilt to stitch diagonially to the corner; after folding I start at the top from the edge.