Well, it depends. You have to look at each situation individually to see what is at stake. If you know that Anne Frank's family is hiding in the office annex, you obviously keep your mouth shut.
In a case like this, it's important to remember that civil disobedience is most effective when it forces the government to mete out a wildly unpopular punishment. What the government has done is bound to be extremely unpopular because it has come perilously close to passing a secret law.
People think they have fourth amendment protections for most of this data, but long established precedent (Smith v. Maryland) is that there is no Constitutional expectation of privacy for metadata on phone calls. When Congress weakened *statutory* protections against collecting call metadata, American citizens *believed* their calling data was still protected by the Constitution. Nobody has bothered to disabuse them of this idea; not Congress (who despite their current posturing passed the law and authorized the program) nor the Obama administration (their posturing on "transparency" and "accountability" notwithstanding). They knew that the majority of Americans had no idea the changes in the law technically allowed the government to run a program like PRISM.
The exposure of this bit of flim-flammery makes Snowden standing up and outing himself incredibly powerful. His doing that means that this issue will *not* die down anytime soon. Look at how long the Bradley Manning case has dragged on, and *this* one, rightly or wrongly, may prove to be far more powerful in the public imagination. I think Snowden might have been morally justified in laying low if he thought he could get away with it, but his outing of himself will keep this issue alive through the next election cycle at least. That could deal a far more serious blow to the PRISM program than quietly leaking it's existence. The cost of that greater impact is that Snowden definitely loses his job, and he faces prosecution and legal punishments.