Hi Judy,
If you are not using a walking foot on your machine, put the bias piece on the bottom. When you sew, the top foot has a tendency to pust the material along, causing it to stretch a bit. The bottom feed dogs tend to grab the fabric and pull it along. The bias will stretch slightly but it might be offset by the stretching of the top foot.
When I have have a piece slightly longer than the one I want to piece it to, I place that longer piece on the bottom. Then they seem to even themselves out at the end of the sewing line.
Make sense?
Connie