BOO - I accept that your comment about you not caring whether customers complain is unclear in meaning and could be taken either way.
However, the remark you made previously, about cashiers having to look at the sour-faced customers while asking if they want a receipt, is quite frankly, rude and uncalled for. Customers are not just being awkward in wanting a receipt!
Pa_ul - Of course it is a good thing if you are asked if you want a carrier bag, for obvious reasons. But it is a completely different matter to the necessity of having a receipt for your purchases. How are you going to prove you have paid if there is a discrepancy? Queries over paying might not happen often, but they do happen.
Another point I can mention is - my sister bought a punnet of raspberries at her local supermarket. The top layer looked great, however the raspberries underneath all had fur coats. They weren't cheap. She rang the supermarket to let them know. They told her they couldn't help her unless she had her receipt. Fortunately she had it and the raspberries were replaced.
It is absolutely common sense to take your receipt, even if it is just to check that you have been charged correctly!