My personal rule of thumb is if the cost/value of the autograph is greater than the value of the prop then its okay. Otherwise just make a custom plaque to get signed and display it along the prop. For example, Harrison Ford's autograph runs around $500-$800 so having him sign a $1,300 Blade Runner blaster wouldn't be worth it for me. A $200-$300 blaster might be a better prop to get signed by Ford. Just my own personal preference on this matter.