I tend to use rewards cards for most purchases. I have a few of them that offer different rates on different purchase categories. By picking the right card for the situation, I imagine my average cash back percentage is well over 2% at this point.
There are only a few circumstances where I won't use the rewards card. One example is for certain bills that tack on a fee for using a credit card (our local gas utility and property taxes fall into this category). These fees are typically more than the cash back percentage I would get for paying with a card, so I use cash.
I'm also mindful of the credit card fees that smaller merchants have to pay. Last I heard, the most common model is for the credit card company to charge the merchant a fixed "swipe fee" per transaction, plus a small percentage of the transaction value. That means they have to pay a relatively huge percentage for smaller transactions. National retailers often have agreements to limit these fees, but the smaller businesses do not. Because of this, I tend to pay in cash at small, independent businesses when the bill is below $15-20. I figure those few cents I'm sacrificing will come back to me in the long run if I do my part to keep more money in the local economy and out of the big banks.