the way to avoid a scene is to call up apple care and create a case # over the phone. tell them there is a dead pixel and you would like a replacement. they will put that in the notes. ask them to make an appointment for you and include in the appointment notes everything that was discussed and that you would like a replacement.
when you arrive at the store, if they give you any problems you just tell them that the CSR said you would get a replacement if you take it in. they always say "OK, sure"