OK, I’ll confess, I secretly stereotype. And when I see a tattoo, the class of the person automatically drops a level or two.
Stereotyping is ~generalizing~, the sorting of observed phenomena into categories so as to make sense of all the impressions rushing our brains. We all do it, and there is no need to do it "secretely," except, of course, to evade the scorn of PC jerks.
We all "discriminate," too...unless we're psychotic.
It is asinine to fixate irrationally on some physical characteristic of a person (race, e.g.) and make strong assumptions based on such an isolated bit of data.
Even race, though, is a legitimate thing to notice. Jesse Jackson was quoted in Newsweek about fifteen years ago as saying that when he is walking down a dark street in a big city late at night and he hears footsteps behind him, he is relieved if he turns and sees that it is a white guy! Incredible comment from Jackson. You certainly can get mugged by a white guy. But the face of street crime in big cities is, for the most part, not a white face. Only a PC-obsessed fool would deny this.
(I know you were being ironic in your remark that you stereotype "secretly"...just wanted to chip in my bit on that idea. Incidentally, if I heard footsteps and saw it was a guy with tats all over his shaved head...I'd be real concerned. And that is more of a white stereotype than a black, I'd say.)