There are no rules! But seams and things usually need to look like you have done things on purpose and have done it well. If you find you have trouble getting all the seams to look exactly straight - try making them look very curvy and wavy on purpose. Anything is acceptable as long as it looks like it is designed to be the way it appears. Straight seams are fine - curved, wavy seams are acceptable. If your seams aren't straight, multiple the waviness until it is a design factor. However - seams that wavy are sometimes as difficult as straight ones. But there aren't any rules, only creativity! Go for broke however you want to!