Definitive guide on where to put radiators?

I'm planning on moving some radiators about so I did a little research on where it's best to put them in a room and as far as I can tell it no longer matters.....really?

When training to be a plumber (or gas fitter) do you actually get taught where to put them and why, and if so has the course material been updated?

It seems pretty conclusive that the reason why you often find radiators under a window (apart from convenience) is because it's more effective as cold air comes in from the windows and helps push the warm air from the radiator into the room itself. However, with the invention double glazing that has now become redundant so I'm left thinking does it no longer matter?

Logic dictates that it might be more effective to stick them in the middle of a room but not sure if any gain is just negligible with double glazing and cavity wall insulation.

Another thing I seemed to often read is that radiators don't actually radiate but convect so now I'm thinking it doesn't matter if I stick one behind a sofa so long as the top of the radiator isn't obstructed. If that's true then I certainly wouldn't mind moving mine from under the window (no longer worry about long curtains) and put it out of the way behind my sofa.

I'm also thinking of putting a new radiator in the small bedroom (possibly tall one) behind the door since that space can't be used for anything anyway.

Are these ideas reasonable or is there some real science behind not putting them in these areas?

Thanks very much and hope you can help.

 

Thanks for your reply,
At least I know I'm on the right track.

Does anyone have a radiator behind a sofa and find it works fine?

Thanks again.