Helinut,
Yes, although the effect is not too large. The downwash velocity varies with the square root of the density, so a 25% reduction in density would result in a 15% increase in the downwash velocity. For a jet ranger, downwash velocity at sea level would be about 1600 ft/min, at 8,000 feet altitude, it would be 1800 ft/min.
On the other hand, the power the rotor needs to hover goes up by about 1% per thousand feet, so the aircraft needs 8% more power to hover. Also, engine power falls of sharply with altitude (not easy to predict in a rule of thumb) but if you lost 10% power on the engine, then a good hovering helo at sea level could have a 15 to 18% power deficit at 8,000 feet.
This would lead some folks to say that it is more prone to "settling with power" but in truth it is more prone to "settling without enough power"