Prism99

What kind of batting are you using? Although technically horizontal lines would be better, honestly you do not need them if you are using a batting such as Warm and Natural which is needlepunched through scrim. It is very stable. Other battings may also be okay without horizontal lines, especially if the quilt is not going to be machine washed a lot, and especially if your quilting lines are fairly close together.

Have you sandwiched the quilt yet? If not, I would recommend heavily starching the backing and top before layering. Starch stabilizes the fabrics so they do not stretch and distort while machine quilting. Spray basting is best for machine quilting because it connects all 3 layers continuously, plus there are no pins to get in your way. Glue basting is a close second. Thread basting is for hand quilting, not so much for machine quilting. Starch and spray basting work to stabilize the fabric and keep it from stretching.

I mention all this because, if you want to do horizontal lines as well as vertical lines, starching and spray basting will help prevent puckers. Without this prep, it is difficult not to get a pucker on the top or backing or both when crossing a previous line of quilting. This is because the presser foot tends to stretch and push the fabric ahead of it; when you get to the previous line of quilting, there is no place for the excess fabric to go, creating a pucker.