Unless there's a discount to do so, I use credit cards for everything. My Amex blue card for everyday purchase for which it offers good rebates, such as grocery shopping and fuel. When it's cheaper we add the minimum number of purchases to obtain 15% off for our Amazon Subscribe-and-Save - 5 items - and use our Amazon card to get an additional 5% cash-back. Things like Muesli, Clif bars, diapers, etc. can be much cheaper if purchased this way.
The only time I pay cash, is if a discount is offered. The guys who recently refinished our hardwood floors complained when I offered to pay with my Amex card, preferring check. I reminded him that they initially said that credit cards were 100% A-OK to use when we were shopping around, and that, when we asked if there was a cash discount (i.e., paying by cash/check) they said that they did not offer one. I compromised a little and used my Amazon card, instead of my Amex, after he said that Amex would cost him more.
I pay off my cards every month, so I don't pay any interest. Even if I occasionally decided to carry a balance for more than one statement period, I'd still make out, since I'm gaining hundreds of dollars per year in cash-back.
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With that said, I try to use cash only for local mom-and-pops places, even if it means going out of my way to draw cash from the ATM.
I also use cash for Craiglist-type purchases, such as for the wheel/tire setup I purchased from someone on one of the Subaru forums I frequent. Saves us the 3% Paypal fee.