I find the best way to quilt a really neat straight line, so it stays parallel to the seam, is to use the Line Tamer ruler
http://fourpawsquilting.com/pages/template.html It makes beautiful straight lines and handles so easily. I use a Sweet Sixteen sitdown quilting machine, and this is my very favorite tool. Found it here on the board.
Thanks for the definition - I didn't know exactly what the ditch is, but I just did exactly that on a quilt I finished off today. Just need to add the binding.