Market timing is easier said than done. Studies have shown that those who sell after a downturn has hit to avoid further loss are about as likely to miss out on a rally as to succeed in cutting their losses. And historically the largest rallies come just after downturns. Not saying that it's impossible of course, but it really is a judgment call. If you see a dip now, do you know if it's just a temporary aberration or the start of a long plunge? If you hesitated and it really did plunge, do you know if an upturn is just around the corner or if the stock is in for a further fall?

In the absence of any clear idea of what to do, holding might not be that bad an idea.

Personal anecdote: I basically held through the 2008 recession. I did suspect that something big was brewing beginning from 2007 but it was hard to predict when exactly things would go bad so I didn't sell. However, I did hold off on adding any more money from 2007, diverting new money into safe assets (like FD and the Malaysian government bond funds) instead. Because of this, I had extra money to spend when I judged that things had settled down enough to start buying. Maybe I could have done better by selling everything the right time and buying back later, but I wasn't confident about the timing and so just held. I'm pretty happy with how I did overall so I don't worry about any potential extra gains I may have missed out on.

Ah, you meant crisis as in personal crisis and emergency as opposed to a general economic crisis that affects everyone. Sorry then. But like others have said, this then becomes a personal financial management issue, not a stock hold/buy/sell issue.