A. It is a good idea to rebalance if the market has dropped a lot compared to your original target asset mix.
When the market tumbles, selling relatively better performing assets and buying into stocks performed poorly can help you bring your target asset mix to the level that is consistent with your risk tolerance and goals.
This is important because "buy low and sell high" is something easy said than done. Just check your own records when you have sold a "winner" and bought a "money loser" last time. A rebalance forces an investor to execute this sound strategy.
However, there is not an universally agreed answer to the question - how often should I rebalance. It's advisable to develop a regular schedule, such as twice a year, to rebalance your investment portfolio, in this way, you can avoid emotional investing.