If you use your credit card(s) like cash and pay off the balance every month, then using a cash back card is like always having a 5% off coupon at the grocery store or gas station, or a 3% off coupon at every restaurant. Other categories with good cash back rewards are home improvement stores and utility bills (if you can pay them with your credit card).
There’s a helpful calculator tool at http://www.creditcardtuneup.com/ that will calculate the best card(s) for your spending profile.