I find my longarm prefers cotton thread. I generally use cotton thread for piecing too, but found some Gutermann poly that needs to be used, so I am piecing with that. Some of the charities that accept quilts for children in hospitals ask that you use everything cotton in the quilts being sent to them. The reason is that the quilts will be washed in commercial washers and dryers and the poly melts in the high heat used. We all find our favorites and no one way is right or wrong, just what we come to prefer.