I don't seem to have a cut and dried formula-I approach each quilt differently depending on the quilting. I am thinking your puckering might be just because part of the quilt is quilted and your border is not yet. I have had this happen to me on a nice flat quilt. I always strive to keep the quilting density as even as possible. I try not to have very dense quilting in one area, and very open quilting nearby. That lends itself to puckers and tucks. When I am changing thread colors, I like to do as much as I can in that area, maybe stitching in the ditch to stablize blocks I will come back to later.