Whirly, the pitot tube is designed to receive a nice straight, undisturbed flow so the ASI can tell the difference between the static air pressure from the static vents and the static+dynamic pressure from the pitot tube. Only when the airflow is straight down the tube, undisturbed by rotor wash and any other turbulence does it have a chance of being accurate.
Unfortunately the rotor will almost always affect the free stream airflow on it's journey to the pitot tube no matter where you place it (this is why you see test aircraft with pitots mounted on a long boom to keep it clear of the downwash).
The pitot system is great for fixed wing that only need IAS indications at or above flying speed but not so good for helicopters that, ideally, would have IAS information accurate from zero to Vne.
Add the problems of sideslip to the equation and you further degrade the system accuracy - then really worsen it by not allowing the airflow down the tube to settle to one value due to turbulence. The result - not a very accurate way of assessing the airspeed of the helicopter, especially at low speeds.