If it won't even get off the ground with full up collective, you should be staying ON the ground.There are some exceptions, however, and it usually involves high speed pieces of lead proceeding in your specific direction
As I described earlier it was also a procedure used in order to get to the scene of high speed lead coming in your direction. One C model gunship landed to pick up the crew of an aircraft shot down which then made them too heavy to take off, by pulling some power they were able to yaw the aircraft in order to aim the mini guns at the opposition and give them a dose of high speed lead. Once the opposition was placated a slick landed to pick up the crew.