If you've got some money you are going to need in the next few months, or are a short term trader, you probably want to consider pulling it out of the market.
If, on the other hand, you've have money in a 401K with a 20 year horizon, you'd be better served to just wait it out and add more if the market goes down. 20 years is a very long time as far as investments go. Think back to 1996. Could you have come close to predicting what was going to happen over the next 20 years then?