I found a website with a tutorial on this and she recommended always sewing in the same direction, not back and forth. She had an example of what happens, how the material can creep just a little bit, causing ripples in the sections when you go top to bottom, then bottom to top. I will see if I can find the website and post a link. I think the idea is that all the creep goes the same way, and then you trim everything level when you're done and you end up with a smoother top.