I like it when I know the person or when they are from the same series or when they are a younger kid or someone not at the con, but when it's a random person it can be a little strange at times, like someone at a con not in cosplay. My friends and I have had a few problems with creepers before and it always makes me a bit cautious, especially when they try to put their arm around me.
I usually won't say no. But it can make me uncomfortable.
Although once my Gurren Lagann group and I were asked to pose with a bride for a couple photos once, that was cool.