There is no problem in a heli taking off down wind, the only things you need to be aware of are a longer run to obtain translational lift and a flatter climb gradient.
Obviously when transitioning downwind your indicated airspeed is going to be lower than your ground speed which then takes its effect on circuit pattern positioning, ground rush etc...
A downwind approach, and yes they do need to be flown sometimes, must be approached with caution. The ASI will, once again, be reading low in comparison to the ground speed and the rate of descent, if flown like a normal approach, will be higher. Always have plenty of power in hand and approach the site a little slower than normal with a flatter approach path. BE WARY!!! The wind coming from behind will put you into the vortex ring area as you approach 0 kts IAS with 20Kts ground speed and you want to arrest that rate of descent
Taking off into wind and then turning directly downwind with little height and a low airspeed whilst feeding in a banking manoeuvre strikes me as a recipe for disaster
