With a tail rotor, the helicopter experiences inherent sideslip (the same as TR drift but in forward flight) so the fuselage doesn't travel along its centre-line with 'wings' level (level on the AI), it drifts slightly to the left or right depending on the MR direction rotation and therefore the anti-torque thrust direction from the TR.
The string will give a reasonably accurate indication of the relative airflow over the fuselage and is often preferred in the slower speed bracket whilst the balance ball will give an indication of skid in the turn but will only remain central on a constant heading if there is a little opposite bank applied to counter the inherent sideslip.