Perhaps..
If you are flying a machine with the tail rotor significantly below the main disc and wish to navigate by DR (typical ppl training in Robbo or S300) then perhaps you should fly on the string so that the machine is flying in the same direction that the compass is pointing. If you fly with the ball then will there not be a greater sideslip element that will not have been taken into account with the DR calcs ?
A small difference, and negligible if you are navigating from one big visible town to another.
I would guess that this difference is less pronounced with types with a raised tail rotor (never flow one).