The top of the curtain (the heading), for all curtains can be at ceiling height, above or at the top of the window frame or flush with the window reveal. It all depends on the space that you have. Do keep your choice constant in your home as it can look very odd if some curtan heads are fixed at different heights to others, especially when you can view through from room to room or worse, in the same room. The finished length of ceiling to floor curtains should leave a 10mm gap, so that they don’t catch the floor and are easy to draw back, the gap for finishing at a windowsill only requires 5mm.
For safety reasons, never cover radiator heaters with curtains, look for another solution, either stop the curtains short of the heaters or use a blind as an alternative style of window treatment. Consistantancy in design of the window treatments allows for a symmetrical form to be created here in this photograph. The head of the curtains are all the same height and the window swag and tail valance details are all the same and they are all held back at the bottom of the window sill which provides a regular frame for the windows and creates a flow and rhythm to the window treatments.
Read more about curtains care HERE.