How should I spend my rewards?

There's a new way to earn rewards at many grocery stores.  For every dollar that you spend, you accumulate points.  Then there are special rewards where you earn an extra 500 points for buying a certain product.  Rewards can mount up fast. Finally you have to decide how to spend your rewards.  You can cash them in for savings on groceries, apply them to your gasoline purchases, or even use them to give to a food kitchen. Many people will choose to give the value of their rewards to charity.  But, if you are going to use them for gasoline or groceries, which way gets you the best deal?