I found that the specialty 1/4" quilting foot caused me more problems than it helped. I have an older Bernina and bought the new patchwork foot thinking it would help me. Nope. It caused me to slightly veer off at the ends of seams for some reason and I didn't have good visuals at the ends either. I found that I still needed a long line accurately marked on the machine bed with the needle to the right side so that the feed dogs have better contact with the fabric. You can use all manner of things to mark your line-- blue painter's tape, 3-M removable mounting squares, sticky-back moleskin, a stack of post-its, etc. I found that I really needed something with height so that I had something to butt up against. Stitching inside the quarter inch ruled lines on graph paper or index cards is great for measuring the scant 1/4" distance. It is useful to check your measurements fairly frequently on sample blocks because things can slip or wear down, and as has been said above, thread and fabric can make a difference.
I finally invested in one of those inexpensive seam guide rulers with a hole for inserting the needle and a long edge to place your seam guide material of choice. The last time I was at the Bernina dealer, I bought one of the screw in metal seam guides for an insane amount of money I thought, but it is easier than frequently replacing tape, etc. when I have to sew a 5/8" seam on clothing. It works on my old Singer 15-91, too.