It is true that there is no hard rules about this. I like to use narrow inner borders as a place to stop and rest the eyes. I like outer borders to from the quilt but not to 'take over' and be dominant. Dominating borders conflict with the quilt and come about because they are not proportional. The ones I have the most problem with are ones that 'just use something I had in my stash' rather than be a real compliment to an otherwise lovely quilt.