Under current law, merchants are not allowed to charge a different price for an item or service when paid for by credit card than when it's paid for in cash or other means. So the optimal behavior in the current system is to maximize one's rewards (especially cash back) by funnelling as much normal spending as possible through credit cards. The card or mix of cards that will maximize your rewards is a function of how much you spend in various categories (e.g. gas, restaurants, groceries, etc.). There is a free calculator tool at Credit Card Tune-Up: Maximize Your Cash Back Rewards that will calculate the best card or mix of cards for your spending profile.