i don't think there's anything inherently wrong with being slightly prejudiced towards people you've never met before. judging people based on their character traits or behavior is normal. it's a defense mechanism of sorts, it allows you to avoid getting yourself into bad situations or crappy relationships.
obviously, being overly prejudiced is stupid. not judging people at all is dumb too, it makes you naive. being a lil bit prejudiced is the way to go, in my humble opinion