Your info is great. I am new to longarming & have my own machine. I am only on my 7th quilt, but I am already seeing what you are talking about with the seams & wavy borders. I am very careful to get my borders on straight with no extra inches anywhere & the last two have had issues. Not bad...maybe an extra inch in 70 inches, but enough to get my attention. After all, I thought I had it perfect. lol Another thing I have found....the outside seams on the quilt top really need to be backstitched. I have had more than one that wanted to open up. Because I am a semi-perfectionist, I have been cutting my borders 1 to 2 inches wider to allow for a seam disruption, but I know that most piecers wouldn't even think of that. I am lucky. It's my long arm & my quilt. I can do what I want with it.
Another thing.... originally when I started piecing, I was very careful to make sure that a seam was ironed one direction all the way. Well, I have done a couple of quilt tops with a lot of seams & have found that if the seam turns it isn't as dramatic a difference as I thought it would be. And sometimes, you have to do it! For me that turn just seems to quilt right in & leave no problems. After some of these comments, I know that if I do a Kaleidoscope quilt, I will free motion all of it, probably making the center of these seam intensive blocks as a no-sew zone...
FYI...being from the Deep South, I am a big fan of Warm & Natural batting. It is light weight, relatively easy to find & seems to "shrink" (I read the stuff about the stretching of the fabric..that was very interesting.) the same as the fabrics & the overall quilting looks good, even if a beginner does it. That slight poofiness it has does hide a multitude of sins. I have found that if I need a seam in the backing, I do try to do it offcenter, that way it doesn't compete with a block seam that might be right down the middle. I use the 1/2 inch seam & iron my seams open. There is so much to learn about long arm quilting that has nothing to do with the long arm. I am grateful for all these posts about the do's & don'ts & what works & what doesn't. Keep it up!!!!!