Very few toppers make quilt borders correctly. Toppers are people who make only the top of a quilt then have someone else quilt it. Most toppers will simply cut long strips then sit down to sew these to the top. Without ever measuring or pinning they sew the long strips along the side of the top until they get to the end then whack it off. What they don't realize is that the fabric is being fed through at two different rates. The feed dogs are pulling the bottom fabric in at one rate and their hand is guiding the top fabric at a different rate.
That is a technique used in garment making to "ease in" extra fullness between two pattern pieces. In making a garment, say for example, the sleeve is larger than the arm hole it is attached to. In order to work in the extra fullness the sleeve is on the bottom so the feed dogs pull it more and the arm opening is on top so the sewer can pull or stretch it to fit the sleeve.
The easing technique really should not be used when attaching borders to a quilt top because it creates a very friendly border. A friendly border is one that waves a lot. Here's my way of attaching borders to prevent the waves. I learned it from someone years ago and don't remember who.
Measure across one end.
Measure across the middle.
Measure across the top.
Average these three measurements together and cut the strips to that length. My average came out to be 41 1/2 inches. I cut two strips this length. One for the top edge and one for the bottom edge.
Lay one strip along the bottom edge. Pin in the center and at both ends. Then easing in the fullness, start pinning the rest of the border to the top. I use lots of pins. I pin about every two inches. Sew these borders on making sure that the beginning and the ending remains even.
After you have the two borders sewn onto the top. Press them out. Turn the top to once again measure the quilt across the top, middle, and bottom but adding the previously sewn borders into the measurement. Average the three figures to get your new measurement. This technique is not the same as it would be for mitered corners.
Repeat the steps for pinning.... center, right end, left end..... and pin in between. If you measure and use an average instead of just sewing long strips, you end up with a top and bottom border of the exact same length. The same is true with the side borders. If the borders are cut the same exact size, you end up with a fairly square quilt and no friendliness.
Without measuring and cutting borders first it's extremely rare the borders end up the same size. Once in awhile someone gets lucky and the borders end up the same size. Fabric stretches and will not behave. Even on grain fabric has a small bit of stretch to it. At least if you cut the borders the same size you know they will be the same size when sewn to the top. You will also make your quilter much happier. Even if that quilter is you. It's very difficult to make a really square quilt if one border is larger (friendlier) than the other.