I myself am a cashier, and it is part of my training to be courteous to the customer. That being said, I personally do not think that I should be as courteous if:
A: the customer is on the phone in my line and is too busy to aknowledge my existance after I say "hello, how are you today"
B: the customer steadfastly ignores me for no good reason at all - we serve you, but are NOT your inferiors
C: the customer raises his/her voice or throws items at me - this has happened and I just left my cash for fear of doing something that could not be easily remedied
I understand that I am part of a company and that I am replacable, however I have a right to not stand for abuse such as the last one in the workplace. I can handle the small verbal arguements (on the customer's side) here and there and the ignorance, however if it is a constant thing I need a time out from these people who do not give a crap about others.
On a side note:
I definately bag for people ;) but again, I find that so many people have so many rules in regards to how they like their items to be bagged, and expect me to read their minds in regards to this that it is almost worth NOT bagging anymore for anyone.
Nothing is worse than having someone stare at you and mumble/groan that you are not doing it right for them, and dissecting your bagging job to meet their standards whilst the line behind them is growing at an alarming rate.
My two cents :)