re: customers adjusting the politeness level to match that of the checkout assistant. Reminds me of that Monty Python sketch "Is this a sixpenny argument or a shilling one?"
I hate it when the checkout lady thrusts all the wet and bloody chicken and meat items in with bread and biscuits etc. before you can start packing. If you ask for separate bags, she tuts loudly. The last time this happened (in Tesco's) I plucked up my courage and asked "Why are you tutting? Raw meat should always be packed separately" She tutted again, scowled and said "Pack it yourself then". Charming!