A. This is a bad idea, especially for young investors with a long time horizon, for 3 reasons:
a. What you are thinking is to time the market. Numerous academic studies as well as brutal facts have proven - no individual can time the market. If you try it, you will get burned and end up learning a lesson and losing couple of years' valuable investment time.
b. Unless you put a large percentage of your portfolio into bond funds, the protection won't be much when a serious bear market occurs, and your long term growth prospects will be seriously hurt.
c. If you have a long investment horizon, what you need is long term growth, not short term stability. You should stay in the stock game, by doing dollar cost averaging, which implicitly means any dip of the market is a great buying opportunity for you.
If you wonder what kind of portfolio you should build that fits your style, please check out the Smart Investment recommendations.