I have to say there is often a PA message to this effect during boarding. There is always space for a coat, even in a full bin, so long as it is in last. That said, the bins are so rarely cleaned, you really are best to use the hook on seats other than the exit rows. Bags have a journey to an aircraft, which can include being placed on footpaths, drive ways, floors of tube carriages, and some of this ends up in the overheads for a long time.....