1 or 2 seams depends on how much fabric I have for backing. Usually 1 seam that I try to get close to the center as possible! I have heard that the seam should be off center since there would be then less stress on the seam. I usualy do not buy 90" backing but rather get 45" and 5-6 yds and cut the fabric widthwise and then sew 45" ends together. A piece of fabric 45x 6yd (216") cut and sewn this way would yeild a peice 90 x 108 which is enough to back my usual size quilt of 80x95. the seam goes crosswise on the quilt. The quilt does not care which way the seam goes. I have a really hard time centering backing that has multiple fabric in it so I try not to do this. Also the way I sandwich my quilts is to lay one edge with a little hanging down on the plywood board I have on my table, clip it down with binder clips, lay batting and top on and pin.(except I is 505 spry adhesive) This means that I do not work from the center out when sandwiching but rather about 2/3 and then I move sandwich and pin the other 1/3. This way I am only moving and re-anchoring the sandwich 1 time instead of 2 times. Easier? try it and see.