I personally have an 80/20 stock/bond split and rebalance between stocks and bonds for this reason - my stocks had recently created an 86/14 split in my IRA and so I sold enough stock to bring it back to 80/20. MY 401k my set-up is automatically set to do that every quarter.
But the general advice, is if you go to 80/20 from 100% stocks, or some other allocation, make sure thats what you do going forward, don't try to time the market, jumping back and forth. Just choose an asset allocation and stick with it going forward.