Depending on how much fabric I have I don't usually care if I put on the sides or the top/bottom first. It does save fabric to put the sides first but I don't always like how that looks, it depends on the blocks I think. Sometimes it depends on the fabric for me too, in some cases I do corner squares. One thing I don't really think about that was mentioned here is about the fabric grain. I know there is a stability issue this addresses but isn't that only if they will be hung? And I also find centers and pin there as well as the ends first, and then work my way in to keep the fabrics even. That does seem to make a difference. But I think you should do your borders any way you like. No quilt police here.

The ancient Greeks built their temples by, in effect, adding the side borders (walls) first and then adding the top border (roof). Most of us have appreciated and copied that "look" ever since. To me, it is still the more pleasing of the two alternatives.

If a rectangle finished quilt I do sides first top and bottom last. If a square with no way up then it doesn't matter which you do first or last as long as you do opposite pairs not like a log cabin unless it is part of the pattern.

Most of the patterns that I have used have been written to apply the sides first then the top/bottom. So I do.

Thanks for your help and comments!

I am a beginner and took a class to learn how to quilt. The instructor had us measure both side edges as well as the
middle and average the 3 values. This number is the size to cut both side borders. She had us do this to square up our quilts, then add the top and bottom borders.

I generally add the sides then the top/bottom just because I like the line across the top not to be broken with a seam. I could miter the borders, but this is easier!

Wouldnt it it be a bit difficult to put up a roof without the walls to keep it up there? Not in construction, so maybe there's a way, but not my forte.......

I add the borders to the side first because of the length is usually much longer for that than the top and bottom border. This saves on the amount of fabric needed to complete the project. Once that is figured out, it is an easy choice.

I miter the borders on almost all my quilts.