We all have prejudices, whether positive or negative, based on what we see in the media and lived experience. What matters is that you remind yourself that an assumption is just that - an assumption. Until you get to know a person, you can't know what they're like and it's wise not to let prejudice negatively impact how you treat them.
That said, here are some of my own:
When I hear Received Pronunciation, I subconsciously assume that person is likely to be arrogant, intellectually elitist.
Midwestern US and Canadian accents give a friendly, down to Earth impression.
Same goes for Hispanic, country accents, and Scandinavian.
"Working class" accents/dialects, e.g. Cockney, ebonics, Scouse/Geordie, give the impression of being straight talking, unpretentious, street smart, and maybe undereducated/uninterested in intellectual topics.
"Redneck" accents make me think of blue-collar jobs, guns and Christianity.
I consciously remind myself not to let these stereotypes cloud my judgment. My own accent I've altered depending on environment; around my mother's family who are more "working class" I speak in a harsher, more relaxed version of my accent; around my father's family, I speak more "formally" and pay more attention to my grammar.
I have always had a passion for language and it's vexing to see people dismiss entire dialects/sociolects because they have different rules. I've been mocked both for sounding uneducated/lower class, and (more often) for sounding "posh". It's no less stupid to throw it at people you think sound uneducated than it is to mock someone for speaking too formally. Language is about communication and people speak what they've been taught. Haven't come across a dialect that didn't interest me somehow. f**k prescriptivism. Everyday speech isn't an English essay.
_________________
“The loneliest moment in someone’s life is when they are watching their whole world fall apart, and all they can do is stare blankly.”
- F. Scott Fitzgerald