Silsoesid,
The scale will read higher when the aircraft is lifting up, since the aircraft is accelerating upward briefly (just as the scale bounces higher when you step on it) but when you are in a steady hover, the weight reading is the same as when the helo was sitting on the ground.
The high temp gasses rsing is a slight bouyant effect but does not change this discussion (it is an interesting side discussion, though!)