I did this customer quilt in early December, and it was perfectly straight. The quilt owner had the idea to have it quilted in perfectly spaced diagonal lines.
I had to line up the first large line going from the top left to the lower right, and figure out how to load it on the longarm. I had tried to figure out how to load it diagonally but couldn’t get it lined up straight enough, so in the end I went with the usual way, and would stop on a line with the needle down, advance the quilt, then continue that line until I had to advance again or I reached the end.
Some lines were 4 advances. Then I would do the next line on the way back up. I did use a ruler for this, one with markings so I could keep the lines evenly spaced the entire time. There was minimal marking, usually on the edges.
I had to double check the line I was quilting again the previously quilted line with each ruler move or advance of the fabric.
It was slow going, but so very worth it.
You can find the strip and flip baby quilt pattern here.