I am more likely to add curves to a straight line quilt than straight to a curved quilt. For me curves add softness and movement to a design. I also consider how many times I want to tie off my quilting or have it be more continuous. Sometimes the only thing that suits me for a quilt is to look at the individual squares with a design and other times I am pleased with an all over design. It all depends on the quilt so I don't follow specific rules. I do try to look at as many quilts as I can to get ideas.