Check the quilts where you got the wavy borders for bias edges. If you have bias edges along the sides, it is easy to stretch them when you put on the borders. Starch the fabric well and if I think the edges of my blocks may stretch, I do a row of stay stitching around the quilt edge before sewing on the border. It also helps to put the quilt top on the bottom so the feed dogs ease in more fabric when sewing the border on top.