I'm always apologetic while using them, and make sure I show them before I reach the till.
Used to work in McDonalds, where it was always clearly stated "Show to till attendant BEFORE placing order", and about 70% of people did, then you had people who didn't realise, and were apologetic (You had to redo the entire order with vouchers), then you had the one or two who were twats about it, tutting while you had to cancel the entire order, then put it back on. 1 meal, easy. 15 meals, bit more of a challenge.
( Agnostic Antichrist Baltimora, Wed 13 Oct 2010, 15:06, Reply)