I mean if you see large run ups and you THINK the market is at a high (who can predict that?), then unless you NEED to rebalance which would warrant some selling (while keeping the rest of your % according to your rebalance (in spite of your thoughts on that asset class being very high)), you would stay vested. Timing the market by "taking some off the table" and say, into cash, can lead to major missed opportunities. I don't have all the answers and have made the mistake of thinking I do in the past. But, aside from rebalancing, staying fully vested is the way to go because of the fact that most corrections in history (going back a long time) have happened in some very short weeks and months. And if you are on the sidelines then you can easily miss those corrections/gains (unless your timing skills are superhuman).