Seams should run horizontal on the longarm frame. However, your quilt can usually be mounted "sideways" to make the seams run horizontally. The exception to this would be if the quilting pattern is directional. I'm surprised that this longarmer wants the seams ironed open. I think they're stronger when ironed to the side, and certainly less chance of batting poking through. It's also not clear to me whether the longarmer wants 5" all around, which is on the high side, or just 5" longer than the quilt top, which is on the low side. Usually 3-4 inches all around is more than sufficient, but if the longarmer intends to trim the backing that may be the reason for asking for 5".