Along with rebalancing, as appropriate, to restore one's chosen levels of reserves, bonds or bond mutual funds, and foreign assets, if after a period of significant market volatility, whether trending upward or downward, an annual review of your assets shows that one of the two U.S. equity categories is significantly higher (ten percent or more) than the other, sell off some of that category and use the proceeds to purchase more of the lower category U.S. stocks or stock mutual funds.
While it is just an option for easy application, one can overall keep 20% each of one's liquid assets respectively in money market or similar short-term reserves (a safe haven in case of a market meltdown, loss of one's job, etc.), intermediate term bonds or bond mutual fund shares, foreign assets/mutual funds, and the two above referenced U.S. stock/mutual fund categories. By periodically selling the higher portions of such an allotment of one's resources and buying more of the lower portions, one assures the basic rule of investing, to "buy low and sell high."
One's annual review and rebalancing (as appropriate) is also a good opportunity for analyzing which stocks or mutual funds to retain and which to replace. Are one's purchase criteria still applicable? If so, no need to make any changes.
If using mutual funds in taxable accounts, it is best not to buy new or additional shares shortly before major dividend or capital gain distributions, since they automatically increase the number of shares (which lowers the price for all of them), and yet one still owes taxes on the distributions as if one had had real gains.