Build a portfolio based on your goals, make sure it is something you are comfortable holding onto in a good year or bad, and stick to it no matter what. If your AA is 80/20 stick to that. Now, to the OP, you don't need cash in your portfolio for buying on dips. That is what bonds are for. Sure, bonds don't earn as much as stocks, but they normally earn more than cash. In addition, when stocks are crashing high quality bonds tend to go up in value. If you had an 80/20% $100k portfolio, so 80k/20k, in a crash it might look like this...
Start: 80k stocks. 20k bonds. Historical return: about 9.5%
At the bottom if you didn't rebalance: 39k stocks, 22k bonds*. 61k AA= 64/36
After rebalance: 49k stocks, 12k bonds. AA= 80/20.
Since the bonds went up when the stocks were going down you had that much more in bonds to be able to throw into stocks at their lows. If you compare any AA with stocks/bonds VS stocks/cash you will see that the stock/bond combination will almost always have higher returns and less risk.
*I based this on the returns of VTSAX and VBTLX from 4th quarter of 2007 through 1st quarter of 2009.