According to Taste of Home, there's a really good reason to always save your fast food receipts.

Insiders say customers asking for a receipt are often an indication that they are working for a business surveying company like Secret Shopper or Gapbusters.  These mystery shoppers need a receipt to get reimbursed and as proof that they actually visited the business.

Because these customers' reviews factor into a specific restaurant's rating, managers will often bend over backwards for anyone perceived as being one of these undercover diners. That means you'll get the fastest service and the freshest food, all served with a smile.  There may even be some extra perks tossed in, too.

Another thing to keep in mind is that these customers tend to make their visits at peak lunch and dinner hours.  Between noon and 2 PM or between 5 and 7 PM.

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