If a guy came to your front door steps, be it a handyman, delivery boy or a random stud who looks hot, would you open the door to them without any clothes on or in see though clothes just for a tease?
I did that once with my girlfriend, just to get her to loosen up and not be such a prude, but even though I'm usually nude around the house, I'd answer the door to a stranger in at least a towel.
When my door is knocked it's more likely to be a child from a neighbouring house than anybody else (if they can't find their parents or they've lost their keys, or there's something wrong and there's no adult about, I'm at home most of the time so I hold keys for everybody and I'm the nearest safe adult) and I really wouldn't think it appropriate to answer the door to them without covering up.
Towel at the very least, but normally a robe is within easy reach anyway.