I had an extra thick ruler made for FMQ with my Bernina. I was working on a star quilt that I wanted to outline the stars and stipple in the background. I just can't FMQ a straight line but I can use my ruler to do so. It is a little tricky but I hold my ruler down on my quilt surface with half of my hand on the ruler and half on the quilt. My Machingers gloves keep everything from slipping. I managed to get my whole quilt done and it worked better than trying to FMQ straight lines without it.
I use my walking foot for sewing straight lines if possible but it is easier to FMQ straight lines than constantly turn the quilt to maneuver around a star.