I wouldn't worry about it. Once it is washed it seems to go away. One suggestion I do have is that when you are quilting straight lines try to first go one way, and then the other way. (EG: From top to bottom of the quilt and then from bottom to top) This seems to keep whatever stretch happens at bay. This isn't always feasible because there often gets to be just too much quilt to fit, but try to do it whenever possible.
You may already be doing this because I don't see the stretch that can happen.
How did you get your lines so even. I always have a hard time with that, so your suggestions are welcome.
amh