I always seam my border and bindings with a bias seam and the seams are not very obvious at all. I also stagger my seams. Rarely do I have a border that doesn't have a seam.
When I do multiple borders I sew them on one at a time rather than make my borders in layers and then sew them on. I measure the middle of my quilt and take this measurement to cut the length and then make my quilt fit that length, sometimes having to ease my quilt in a bit to fit. My quilts always lay flat and are square when I do this. I'm curious as to how you measure a the length of a border with multiple layers and still keep it from stretching. I'd like to know how others do it.