I can say something with all the polite words but a sarcastic voice and it's not very polite. Likewise, I could say "Pass the salt?" with an uplifted tone, a "polite" voice and an ingratiating smile and it's polite.
Interrogative clauses like this are used as directives: I'm asking you to do something. The past tense "could" (or "would") sound more polite and diffident than present tense "can" (or "will").