>I prefer to do chest/back together & bis/tris together....so,
>my ? is, which day is best to do shoulders
I'd do shoulders with bis/tris (do shoulders first, then bis/tris, in that order). Chest/back are large muscle groups that can take more work, while bis/tris are small muscle groups that don't need as much.
Just make sure you don't do the two workouts (back/chest, shoulders/arms) on back-to-back days, as shoulders are also worked during chest and back work and would need more recovery than that (at least for many, if not most, people).