Should I say "sir" and "mam" at a job interview?

should I say "yes" instead of "ya" is it bad to over dress or would it increase my chances of being hired? I think it is important to address the interviewer as "sir" or "mam" but my friends advised me not to because they think it would make me sound like I am desperate for a job and that I am a kiss up. What do you think? I am applying to be a bagger/cashier at super markets and convienience stores.