I always cut my borders to fit, pin, pin, pin, use a walking foot to sew them on, and I never have a problem.

When I was renting time on a LA, the lady that trained me told me that when she gets an unsquared quilt she can do an all over pattern (with the computer) and simply adjust the pattern for the uneven portion when she gets to the bottom. To a judge's eye I'm sure it wouldn't be acceptable, but to the average person I'm sure they don't even notice if the pattern is a bit wider or longer at the bottom. I don't know if this would work for waves, but if it's pulled a little tighter as it's being quilted, maybe?

I don't measure in the center or on the ends. I use a piece of border that's longer than the top and cut it off to square when it's sewn on. If I'm doing a patterned border I'll pin it so it's just right. I have a Pfaff though, so maybe the IDT helps? I've been told by my trainer (who does lots of people's quilts) that my quilts are unusually squared and perfect. Ha! what a silly lady!

The problem is that the boarder is stretched while seing them on if you arent careful, I still feel the best foolproof method is to measure and pin carefully, its much easier to take a few extra minutes and steps and only have to do once ...

I would remove the borders, and start over. What I do (if I do not have a surface big enough, can't use the floor anymore), but I now do it for all my borders is:
fold your quilt in half, finger pinch a fold in the exact middle. Do the same with the border. Match the two tiny folds and pin from center to end. Then sew from center to end. Do the same with the other center. Use a walking foot also. It helps feed the fabric evenly. Also try starching the border to make it stretch less before you start.

As long as I sew with the border on top and the quilt on the underside, everything is ok.

I cheat. I smooth the border until flat and where the excess is I pleat it and sew it down with an angle seam. No one has ever noticed and after quilting it's not even noticeable. Another tip is to use adding machine tape instead of a tape measure if you do measure across the middle and ends. The paper won't stretch. Cut it the length and width you need.

I agree with dunster & BMP- I have had quilts come in that look good until I get down to the borders, have taken them off myself and fixed because I did not want them thinking that I did a lousy job quilting. Gave them instructions on how to keep that from happening again. Measure and measure again, pin, pin, pin. In the end you'll be glad you did.

What I do is measure the center of the quilt. I use a measuring tape that doesn't stretch. I cut the border to this size I then find the center of the quilt & the border & pin. Then pin the ends then work in the center of each end. Do the other side with the same measurement. This should make each side the same.

I am afraid I too would take the border off and redo it.

I have always done the three measures and then averaged them for the borders. Recently a friend who LAs for her business told me she ONLY measures the middle. Most of my quilts are pretty close but occasionally I get one side that off a bit. Here is what I do

Determine how long (and like PatriceJ, I prefer length of fabric to width because of the stretchyness -not even sure that is a word - lol - of the woof threads) and cut two identical pieces.

Pin both ends, then the middle, then the middle of that (you get the drift - lol) ... If there is extra fabric anywhere - ease it in carefully (I use lots of pins for that). If you have one edge where a lot that needs to be eased in, you can use the old seamstress trick - run one line of stitching just inside the seam allowance with a loose tension - then like you were going to gather, gently ease in the fabric you need . Then sew.

Good Luck

I agree; if you do it this way you can ease it in if needed. I start pinning the border onto the quilt in the center and work my way out to each end. It is so easy to stretch fabric, even if it isn't on the bias if it's not pinned and you are just guiding it along. Also, cutting on the bias isn't needed unless you are doing curves. I normally never cut on the bias. The boarder and binding take a lot of abuse so if you can cut it on the straight of grain it will be more stable thru out the life of the quilt. I'm much rather take a few minutes to measure and pin than trying to fix a wavy border or even to deal with it.