I have always sewn my borders on an angle. I was taught this way, but also because of the direction, it is supposed to stand the test of time and usage better. A straight seam is easier caught and torn, than a diagonal seam. I guess we'll know for sure in 100 years, which seam lasted longer.
I have, however, done both types of seams, and who knows if it matters. I have always taken my backing to the LAQ. Sometimes they have batting and sometimes I provide my own. Just depends on whom I use.