When I make ruffles, I always cut the fabric on the bias. When cut on the straightgrain, the edge usually shreds and the edge does not behalf well. On the bias, this does not happen. I also connect ruffle ends that have been cut on the bias.
For example, when making a pillow top and adding ruffles, I mark the ruffle length in fourths. These fourths correspond to the the pillow top corners. I zigzag over a quilting thread using monofilament thread. Align the fourths markings and then draw up. Doing this will allow the same amount of ruffle to be even between the markings. -- Also, allow extra fullness at the corners to avoid the cupping thing.