Capital controls at this point in Greece are extremely unlikley, and were never all that likely in the first place. (It was politically-motivated rumor-mongering.) During the time of the Cyprus crisis, when capital controls were imposed (and still exist). ATMs always worked, and while withdrawal limits were capped, they were not capped at levels so low that an ordinary tourist would be inconveniced.
As a tourist, it is unwise to carry around more than 300 euros in cash, and please bear in mind that the real reason for capital controls is to prevent cash from leaving the country. So if you elect to bring in euros because you fear capital controls, don't bring in more than you can spend, and don't keep topping up at the ATM machines.
Also, if you pay cash for hotels, get a receipt.
But generally speaking, the best strategy for people worried about capital controls is to rely on using your credit cards -- and again, capital controls were being threatened as a political tactic, and the events of the past few days have shown that tactic didn't work. Capital controls are less of a possibility today than they were 2 weeks ago.
What bears watching as potential traveler to Greece over the next weeks and months is actually civil stability, and even past experience shows that public protests in Greece seldom have any noticeable impact on tourist areas.