I'm thinking Percy and Dean might have had a few run ins when Percy was a prefect, nothing too serious but maybe a few lost points and a talking to for Dean. Dean was probably a bit eyerollingly impatient and thought Percy was a stick-in-the-mud or something similar but less polite.
Since Hogwarts... I'm guessing the pub is their main point of interaction unless he's got a hidden interest in art! Dean's probably amused by how vicious Percy is about the quiz and glad he's not generally caught up in that. Happy enough to serve him his drinks and exchange a few words when he's working.