The dangers of hitting the ruler with the needle are even worse than throwing off the timing. I don't have experience with ruler quilting on a DSM, but I do it all the time on the longarm. If the ruler slips under the foot, it will break the needle, so pieces can fly everywhere. Think about everywhere including into your face. It's a sudden and scary event, which may cause a hole in the quilt or worse. I would invest in the correct foot, read or watch instructions for how to use the foot properly, and even then be extra careful.