A general rule (but not hard and fast) is if the quilt top is mostly straight lines, go with some curves in your quilting. And then the opposite should work too. I'm a longarmer so usually the customer will give me a good idea of what they would like. I then show them various designs I may have that encompass this and we discuss it. I do get stumped sometimes, so I will put the top on my design wall and study it at length usually all my books, pantos, etc.