My goodness. I never thought about it before. The first quilt I made (now owned by my sister) has sashing and a couple of borders. I just cut and sewed. Now that I have done quite a few quilts, I usually do whatever happens to be the shorter sides first, if it is a rectangle.
There have been a few of my quilts that I did not bother with borders. Do not stress your self over them. What ever pleases your own eyes should be fine.