Most are and a heating contractor gave me the explanation I recounted in post #31. I guess YMMV. If there is no register under the window there will be more cold induction from the window.
Ideally a window will be fitted with a curtain heavy enough to shut off most air circulation against it when the temperature differential between the outdoors and the inside is great.
No, the absence of a register (or blocking it off from exposing the window to its draft as with a table) means LESS induction from the window, because the air is stiller.
Heating contractors repeat what they’ve been told. They do not start from first principles of physics.