The heaters need to be exposed to the room to allow it to heat them.
We have curtains that are long enough in most rooms to tuck behind the radiators in the winter. This means they are long enough (I don't like curtain hems which sit on the window sill). In the summer we just shut them.
I have always like the look of big curtains sweeping the floors. But my heating engineer husband taught me that the best design for central heating with radiators put the radiators under the windows. The cold windows help for convection process for warming the room - and I have found this to be true i real situations.
Eventually I hope to look forward to a future of long sweeping curtains and no radiators just underfloor heating!
In the mean time, yes you are loosing heat if you put your heater behind a curtain (or behind a bed or behind furniture...