Now I always thought the Autograph question is Columbo's way of finding out if the suspect is left or right handed. I think he's done that on more than one occasion. I'm probably looking at it too deeply.
The only episode I'm aware of that being left-handed came into play was Death Lends A Hand. Although with as many people that were killed by someone hitting them, I'm surprised he didn't check more to see which handedness matches the side the wound was on.
I think Columbo sometimes likes to ask a socially inappropriate question just to keep the suspect off-balance, see how they'll react, or to make them think he's a rube. In this case he combines it with the flattery of asking for an autograph, another Columbo move, to make them think they've got the upper hand (although I agree with Rob that it's also to see if they're left-handed).
I suppose he could have been checking to see if Benedict was left handed or not but in the end it didn't play a role (as I recall; haven't watched this episode for a long time).
It's funny speaking of autographs. Columbo never asked for Nora Chandler's autograph. Yet he did ask for Ned Diamond's (Forgotten Lady) but NOT Grace Wheelers. Just an observation.