On my sewing machine it is 6.5 that is a close to perfect 1/4" so that is what I use. I don't have the option for 6.3. As long as I stay consistent on the seams and the quilt is for me and not a swap I am good. If I have to change things up for a swap I then use a seam guide to insure that I have things just right.
The majority of the patterns I use are in inches and not metric. Many have both systems for those on the metric system.