I'm installing ceiling heating registers. Are they required to be a certain distance from the wall?
Place them wherever the cold comes in. Under windows is preferred, but if it has to be over them, well so be it. Same for exterior doors too. The principle is if you put registers too far away from the cold source, one side of the room will be cold, the other will be warm. So stop the cold at its source.
Now to answer your question. Since window treatments are always part of a design, make sure the registers are out far enough so they are not blocked by the window treatments.