Hard hats should be worn if there is a risk of something falling on you.
I guess, cleaning windows, somethin may fall on you, a loose roof tile perhaps.
Gutter cleaning. Well your head height is normally above the gutter. So perhaps the chimmney stack may fall and hit you, but then i'd be more worried of falling off the ladder.
So, unless your worried about a cloud falling on your head, or a bird poo'ing on you. Then i wouldnt think a hard hat would be neccesary