How far away from the wall should ceiling heat registers be?
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.
7" has been the standard for all the jobs I had worked on. The only exception has been when a designer has spec'ed heavier than normal drapes, so they would be set sometimes out as far as 9"