I do agree, you have to do what you want, and what makes you happy.
However, personally I only go for Main cast, Director, or any other important figure connected to the film. As an example I had the Main 4 cast sign my Dawn of the Dead Quad. Also George Romero, and Tom Savini. I had the chance to get the others in minor rolls who had played Zombies, but I drew the line there. If I wanted their signature I'd have got it on a still or something, which can always stay with the poster.
I usually think, I or my kids may need to sell these at some point...