I always use the same thread in the top and bottom when I piece. That way no matter which way I have to press a seam the thread takes up the same amount and doesn't cause seam differences. I always use all cotton to piece with, either Aurifil or Prescencia. They're both fine, low lint threads. Poly thread can melt so I don't use it for piecing. I don't always use the same thread weight or color top and bottom when quilting though. I have a nice selection of all cotton quilting threads to choose from. If a poly thread suited the project I would consider using it for the quilting.