If you do the side borders first they would not extend past the top & bottom. You put both sides on Then measure and add the top & bottom

The Pa. Amish quilters do the sides first then the top and bottom. But I think the judges at the big shows prefer to have them mitered which is hard for beginners but it is something you can get used to. I don't really care if any of my quilts get into a quilt show or not. To each his own.

Me too, it just looks better to me, with the sides done first.

After reading all the comments, I agree with the fabric saving. However I thought that one item was left out. I sew sides on first then top and bottom borders. The reasoning that I have is that you pull the quilt up under your chin by pulling on the top border of the quilt. By one piece construction, the border is more durable. I also longarm quilt the border a little more densely for the same reason.

Like most others here I do the sides first but I suppose it a personal choice. The more important point is to take measurements, not just sew a border on and hope for the best.
Divide both the quilt top and the border into 1/4s and match up the marks. Do this and you'll always have flat borders providing you have measured properly. Measure twice(three times if you need to) cut once.

I always do the sides because I like the look of the finished border better. I always measure top to bottom in the center and if it is a larger quilt I'll also measure quarter way top to bottom and take an average. Once the side borders are on I repeat the process for the top one.

I don't understand the comment of being easier. To me they are both the same process and one is as easy as the other.

Same here.

For almost all my quilts, I sew the sides as the first borders and then do the top and bottom. I am like most others, I just like the look better. If you are using cornerstones, it doesn't matter, neither does it matter if you are mitering your corners.

If I am doing a folk art type, then putting on the top & bottom first just adds to the charm.

I think the sides first and then the top and bottom looks best. However if it is a problem for you, make a corner square and then no one knows which came first.

I've always been told that if you want you're quilt to look longer sew the top and bottom on first if you want it to look wider sew the sides on frist