I'm not sure that judging people on poor grammar and spelling feels justified to me, per se. As another commenter mentioned, though, I tend to judge the people who proclaim to not care.

I've also been known to judge negatively people whose words are incongruous with their actions, people who slurp or make loud smacking sounds when eating, and people who stand in the way of passengers walking out of a train/bus. Justification be damned. :) (But it's good to be aware that I judge these things.)