I prefer the diagonal seam. Your eye tends to flow over it instead of being stopped by a solid line. And when you quilt it you'll have to look hard to find the diagonal seam later, it practically disappears.

I let the material tell me. My personal preference is diagonal seaming, but some prints look better with straight seams.

The answer could also depend on the quilting design you're going to use.

I usually do diagonal on solid-color fabrics and straight on prints where the seam is less noticeable. When I do the diagonal, I make sure that I have all of the diagonals pointing in the same direction all the way around the quilt.

I always did the diagonal until I made the Lover;s Knot quilt by Eleanor Burns. She said in her book to do the straight seam, so sometimes I join the borders like that and press the seams open. Since I do FMQ on my quilts I'm sure they will last as long as the quilt.

I depends. I do both. It often depends on how much fabric I have for the outer border. I like to make 8-10" borders which takes a lot of fabric to do the diagonal. If I have the fabric then I will do diagioal but if not then I do staight. My eperience has been that it really does not make much diffrence. For inner borders that are smaller then I do diagonal piecing.