I'm in flux on this one at the moment. I got in the habit of using plastic all the time, debit card, and that was fine. But then I went back to my old Boy Scout ways, keeping some cash on hand for smaller purchases or where they don't take plastic. Also, it's nice to have cash when I find myself someplace where I don't trust them with my credit card.
Going forward, honestly, I am hoping to get out from under all my plastic and try it the other way around. I want to carry cash for everything, and only one plastic in case I'm in a pinch. I feel like cash is more genuine? Like it will keep me more honest about what I really need, and what I'm really paying out for it. We'll see how that goes. =D