It's also important to remember the purpose of rebalancing. It's to maintain your asset allocation because you chose that AA based on your desired balance between risk and reward. So you only rebalance periodically, with contributions and/or when it moves outside of your AA by a %, as per your poll options. (You shouldn't mix and match these too much. As others have said, buying with contributions is best - you probably have a very small portion of your AA reserved for cash, so as you get more cash, you move it into the appropriate lagging fund.) You shouldn't change your AA because the market conditions changed, nor should it trigger a rebalance unless you settled upon a drift % strategy.
I found that even with the 10% drop in equities, due to my 401k contributions being straight equities, and my 90/10 AA, my drift was still very tiny, so there's no real reason to rebalance. (I still did a tiny rebalance inside my IRA to get my international bonds back down to 3% and my Total Stock closer to 70%).