I generally don't take a receipt for groceries - unless I've bought something in the "middle-aisle" that I might want to take back if faulty.
The thing that most bugs me when the pleasant cashier asks, "would you like a receipt?" is just how paralyzed with indecision some people become over such a simple question requiring a quick "yes please," or a "no thanks." I regularly find myself stuck behind someone who, when faced with such a major life decision, goes through all kinds of emotions before blurting out a response: "eh, eh, ah no thanks - "oh eh on second thoughts maybe I'll take it, just in case."
Was tempted to ask a middle-aged women recently would she like to phone a friend for help with the answer...
(On a similar vein, I heard the motoring journalist Philip Nolan saying recently that an increasing number of drivers when stopped at the red light, act as if the light eventually turning green has come as a huge surprise to them, and an unexpected event that they weren't prepared for.)