I was always under the impression that, if you have nerdy interests and someone else that you know has nerdy interests, it's something that you have to gradually build up to over the course of standard social interaction, also known as making friends, that kind of thing. If you literally just waltz over to someone and blurt out a nerdy interest, then you're basically flipping a coin at best. Either that, or you're at some kind of fan convention.
In other words: suddenly revealing a nerdy interest to someone you don't know is very much unwise. In the same vein as politics or religion; it's not something you casually talk about. In an ideal world, I'd extend this to what sports team you support, but for some reason sports fans get a free pass despite basically being a different kind of nerd.