Actually if I were in Tokyo I would try to leave. Japan is quite good in dealing with earthquake (at least the architecture is pretty safe) but radiation...I would be scared too (if I'm in Tokyo).
I think it is a good precaution to evacuate people orderly and early before the situation get out of hands and it is often already too late if the government is taking the situation responsibly.
Whether people will panic is much to do with whether they are still trusting the government and holding confidence in the ability of the government dealing with the situation.