Rarely do I let a person's appearance dictate how I feel about them as a person, even if that's the first time I've seen them/meet them.
While it adds into the information about a person, I don't like to dislike people on looks alone, unless they look, act and dress in a way that I disapprove of. The key here is that their actions are graded the most, looks and dress are low priority usually, unless the look and dress supports their actions. (When I say look, I'm referring to body language and other non-verbal cues)
I've been known to silently observe a person until I'm sure they're a good person, and then introduce myself to them. I do this a lot online and offline, and usually someone has to put in a good word for you, or I have to catch ya red handed in being a good person. xD