I do all mine at an angle.

you won't see it after it's quilted..........

I've never joined borders on the diagonal unless it was for a mitered corner. Sometimes I work hard to match a pattern so the seam is "seamless" but it's still a straight joint. Binding is always 45 degree angles.

I usually join at an angle when there is enough fabric. However, with a very busy print, the seam can barely be detected either way.

I usually sew my seams straight because I like wide outer borders- 8-10" and sewing on the diaginal uses lots of fabric. I usually sew my thinner inner borders on the diaginal. I think that either way is not noticible after the top is quilted.

Usually when I was making queen size quilts, I would buy 2.5 yards and rip my border and binding the cut length of the quilt. But for shorter ones, I also just tried to match up fabric print the best I could doing straight seams.

I hope this shows up. I have a friend from S. Africa who was visiting me when a box of fabric came in. She loved it so much that when she married a little later I made her this quilt. This is Bonnie Hunters Scrappy Trips around the world - not so scrappy. I designed this in my head and when it came time to finish the border I had just pieces left - oh no, what to do. I did have a yd piece of stripey border so I fussy cut certain sections out and attached it to my leftover border and by golly it turned out great. Sometimes design decisions just happen and this is one of them.

I, too, try to buy enough fabric and tear the borders so I don't have to piece. However, it that does not work, it depends on how much fabric I have and the pattern of the fabric. Stripes, I always piece straight. Solids and prints I try to piece on the diagonal. I generally press my seams open to reduce bulk. If I do scrappy binding, I generally piece straight.

For me, it really depends on how wide the border is and what the fabric is. If the border is narrower than 3 inches or so, I will do the diagonal join. But on a wider border, or a plain or tone-on-tone, I prefer the straight join. That seam can get pretty long on wide borders or obvious in plainer fabrics. JMHO.