It is not a good idea to tuck curtains behind radiators when the heating is switched on. The typically thick fabric that is used in the makeup of curtains will cause panel blockages at the back and sides.
And given that radiators create and transmit heat from their entire profile, coverage of any part of the device will lessen their potential heat output and stop the room from getting to the temperature you desire.
For further information, explore our blog, Curtains over radiators: All you need to know.