I've decided to partition the 17ft.+ guillemot s&g into three scarfable sections, approximately, 6ft.- 5ft. - 6ft. (6-6-6 just didn't sit well for me for some reason).

Is there any advantage in choosing another combination, say 8'-8'-1' or 4'-8'-5' for instance?

A deck with 6'-5'-6' pieces puts a joint between the rear hatch and the cockpit, with 5' for the cockpit area, then a joint, then the front hatch.
The hull bottom has a joint directly amidship. The side will probably be 4.5'-8'-4.5' just to offset the joints in the adjacent panels.