If you DCA into a blend of ETFs it's important to keep them balanced according to your established mixture. Two methods I use are Threshold Rebalancing and Inflow Rebalancing. No matter what you do, do not rebalance every month or every move. I rebalance quarterly and will go out as far as bi-annually if the market is particularly choppy in a given year.
Threshold Rebalancing
If an ETF jumps or dips outside of a specific allocation threshold (5%, e.g.) you want to sell the other overweight ETFs and buy more of the underweight. For example if QQQ rips to 30% of your total portfolio value, sell it back down to 20% of your portfolio by dollar value and buy more of everything else in your basked to keep ratios even.
Inflow Rebalancing
When you DCA, just DCA selectively to rebalance the $ value percentages that your ETFs are spread between. This is often ideal for a rebalance that is close to a DCA period because it does not cause a taxable event.
Performance
I have no idea if the returns are meaningfully different depending on which of these methods you choose to skew toward. This is just a snippet from my notes that I'm posting here to get this site going.