When I flew Cobras in RVN I found that when firing rockets in a dive that if you held the slip ball 1/2 ball width out of trim to the left the rockets would fly parallel and true on the sight. Obviously, this was because I was correcting for the inherent side slip.
I have used the string on a couple of occasions and during tests with a boom and side slip vane out front several feet I found that most of the time it didn't agree at least in Engineering terms. The string on the wind screen is subject to all sorts of conflicting airflow due to unsteady rotor wash, particularly near the best rate of climb speed and lower. So.... I'm convinced that the string is better than nothing, but not necessarily the best indicator for side slip. Maybe if the string was calibrated with windscreen marks against a yaw vane, it might be more valid.
Just my two cents worth.