When I am joining fabric for width if I can do an angle I do, not only is it less noticeable but in my opinion more stable than a "butt joint".
If it is a border then I look at the rest of the quilt and also what I'm using for my fabric. If the quilt was all butt joints and no angles, then unless I was working with a stripe I'd most likely put the long sides on first, then the top and bottom.