Depends on the item. An average TV Guide form the 90's might not fetch much with Shatner's picture on it, but his autograph would probably make it valuable.
When it comes to collectables like action figures in mint packaging and such, my experience is that the autograph will hurt the resale value.
I like prop reproductions. Some licensed and very expensive ones come with displays and little plaques that Trek actors have signed. Too rich for my blood. Other people sell fan made prop reproductions or assembled and painted kits. If someone was selling a Motion Picture era tricorder repro, having an autograph on it would kill any interest for me. In fact, if I came across something like that cheap, I'd do my best to remove the offending autograph!
And nobody should ever have an ACTUAL screen used item signed by a Trek actor! That's just a crying shame!
Then again, whatever people are willing to pay for is what goes, but my interest in collectibles says its not the same as sports stars signing game used balls/pucks/etc. A signed Babe Ruth baseball he hit a home run with? Awesome! A Shatner signed original screen used phaser! UGH! Don't cross the streams!