Done it both ways and can't really see a difference, other than that going "middle/middle" is easier to keep straight in your mind as you build. I went to great lengths trying to control exact variations in upper vs. lower limb length for awhile, then an old timer told me that moving the nock point on the string 1/8" would more than compensate for any of the variations mentioned. I quit worrying about it. As builders, I think we tend to obsess about relatively minor points, or, as the sign in my shop advises me: "A bow is just a wooden spring that throws a stick. Try not to complicate it."