If you have a budget and come in under budget each month, then a rewards card can make extra money each year. I don't spend less because I have no spend days. I don't spend more because I use a credit card. I track every expense from bridge tolls, to $1 used books at the library sales, to the occasional frozen yogurt stop. I follow that budget very closely and usually come in under budget each month.
You have to do what works for you, but the aspect that manages our spending in our family is a closely followed budget, not what days I spend the money on or whether or not I pay cash or use a credit card. If you can get a discount for paying cash greater that your credit cards rewards then that can be a good deal, but I doubt you can get discount for paying cash for 100% of your expenses, especially at places like Costco, Kmart, Safeway or Verizon.
If you make an extra $400 a year in rewards points that can be the equivalent to earning an extra $600 or so in taxable income depending on your marginal SS, state and federal tax brackets. If you can get $400 a year in rewards points that would otherwise take $600 of taxable income to equal, then over 30 years that could be equal to earning 18K.
If you don't track expenses and don't pay your cards off each month, then having the rewards cards might cost you more in extra purchases and interest than you would save in rewards.