You're right, it is hard to find any guidelines, and I think that is because there are no right answers to this question. It's a matter of artistry to decide what to quilt. There are books that show the same pattern quilted different ways, or that show lots of patterns, and there are classes that are designed to help you be better able to pick out designs, but still it's all up to you. Think of it as if you are looking at a canvas and deciding what kind of picture to paint, but someone has already painted a picture (the quilt) on the canvas, so you are adding to it.