The goal of Modern Portfolio Theory (MPT) is to maximize return while minimizing risk. Harry Markowitz wrote a book on the topic and in 1990 received a Nobel Prize for his work.
When an ideal asset allocation is determined and implemented, it is usually a benefit to periodically re-balance the asset class percentages. Re-balancing is a way of selling high and buying low. Because different asset classes grow at different rates, and perform differently over an economic cycle, periodically re-balancing usually produces better results. Some custodial accounts have an automatic re-balance feature.
The big question is how often should I re-balance asset classes. Quarterly is probably too often. Surprisingly, there is little difference between re-balancing annually, every 2 years or every 4 years. Some writers suggest re-balancing when the percentage of an asset class deviates too much from the planned asset allocation. Vanguard has a downloadable PDF titled Best Practices for Portfolio Rebalancing.