That seems to be the advice a lot of people are following this week. Nobody, and I mean NOBODY, knows when the bottom of any dip will come. Next week, or by Christmas, the market could be down another 15% or up 15%. There's bad news and there's good news and this week the bad news is getting all the headlines. That means, if you have cash, a buying opportunity is either present or near (I'm holding off a little longer).
In real life I was up 30% early last week and now I'm up only 20% for the year. Ouch! But I haven't made any changes. In CAPS I've just done some minor tweaking.
If you have stocks, then if they were good long-term picks a month ago, they are still good LT picks today. Hold. Why would you buy a stock if you thought it was something you should dump if the market dips? Maybe this dip is telling you that a few of your picks weren't so bright after all- go ahead and dump those if you think they won't be higher six months from now than they are today (especially the ones that were underperforming when the market was going up and are tanking now).
Otherwise, sit tight. If your emotions "can't take this beating any longer" - just walk away for a week or two- pay no attention to your portfolio. You can be an emotional investor or you can capitalize on the emotions of other investors. Your call.