I have a 6600 and with my quarter-inch foot, I set my needle to 4.2 and get a really good quarter-inch seam. It depends on the foot, too - I have a second quarter-inch foot that is from another machine and I can't mix the two up or I won't get a good seam.
And then if I switch to my Accu-feed 1/4-inch foot it's a whole 'nother setting that I can't remember off the top of my head!
Whatever works...works.
