Is it better for heating to close curtains or keep open?

[vicars]

Hi, we have an older Victorian property that has a 3 part window at the front - big middle pane and 2 smaller sides. The radiator is in the middle and I was wondering if it is better to close the curtains fully or to just close the curtains to cover the 2 side windows and get a blind for the middle? Assuming we can find something that looks good is it better to not cover the radiator or close? The reason I ask is that I always remember being told that heat rises and at the top of the curtains it circulates into the room. But is too much heat trapped by doing this to make it uneconomical....?
Look forward to replies, thx.

[Ethel]

Have your curtains long enough to cover the window but short enough to go behind the radiator; and close the curtains.