When I first got DCS:BS I found the helicopter to be too sensitive aswell. I flew with a slight curve for the first few weeks. After that I switched back to the standard settings, no curves and no deadzones at all. Helicopters do require small and precise inputs, and it takes time to get used to. I use a X52pro by the way.
Anyway, I have no experience with other (older) sticks than the X52pro. If the stick doesn't allow precise inputs (no matter centerplay, the X52pro has some, but isn't a problem) I guess you need some curve.
My advice is to use some curve and reduce it over time. As the helicopter felt too sensitive in the beginning, it feels perfect now. In other words, training reduces sensitivity aswell.