Being a "product" of the 50's I was brought up to have good manners e.g. stand up on busses/trains for senior people, pregnant or the less able; open doors, speak politely to all and sundry etc.
Working in my parents corner shop/offlicence (Open All Hours - I was Granville before he had been created!) I know the challenges retail employees face but still do not accept that rudeness should prevail form either side of the counter.
Being polite and friendly to everyone is the best policy because it does help engender good behaviour all round and helps your working day go better.
Being "nice", however, can sometimes have its "challenges". I remember one lonely, middle aged lady who used to come into our corner shop every Saturday and buy the same things each time. She would always wait for me to serve her even if I was busy and there was someone else availble to assist her. Being young and having more memory cells than I have now I used to assemble her weekly order without barely a word mentioned between us. It was a mutuallly happy arrangement - she was pleased to see me and felt cocky being able to produce 20/30 items for her without being prompted. Eventually, I did something silly like getting married and moved away. A couple of years later I happend to visit home and found myself helping out in the shop when who should walk in but this lady. She waited for me to serve her jsut like before. Unfortunately she expected me to remember her order - suddenly I was not so cocky ........