When I pin 2 strips of fabric together, I start by pinning the ends first, then straighten the strip out and put a pin in the middle. If you find that you have more material on the top or bottom I'll put an extra pin between those sections, too.Then when you sew them together always put the strip side that has more material down. The feed dogs will help distribute the material more evenly. Also, be sure you are not holding your material too tightly or pulling it too tightly, remember you are only guiding the material through.
I also have a plastic seam guide that I can screw on the machine so that it helps to keep a scant 1/4".
Good luck.