I know you've found your solution, knlsmith, that's great!
But for anyone else who is doing a project, and does not want to pre-shrink the cording ... if you can leave the cording "free-floating" inside its fabric tube, not catching the ends into your seams, then the worst that would happen is that you would have some empty spaces in the ends of your piping tubes. And IMHO that would be better than puckers.