Check out how pretty straight line quilting (not in the ditch) can be from a favorite of blog of mine:
http://www.redpepperquilts.com/2009/10/tutorial.html
This is what I do--just chalk mark straight lines either diagonally or horizontal and vertically. I don't pay attention to where my seam lines are--I just mark the quilt in lines usually 4 inches apart. I follow those lines with a lengthened stitch with a thread like Aurafil--either white, color matching quilt or a variegated thread. This makes for a nice all-over grid--looks nice front and back. I think stitching in the ditch is difficult.