A. Yes, it is not only desirable, but also recommended, for the following 3 reasons:
1) Tax-loss harvesting means you sell a stock that has lost value, that could generate losses to offset your gains.
2) By replacing the losing stock with an ETF, you reduce overall risk exposure of your portfolio.
3) You avoid the Wash Sale Rule, which states that your tax loss will be disallowed if you buy the same security, a contract or option to buy the security, or a substantially identical security, within 30 days the date you sold the loss-generating investment.
Sell one ETF and Replace with another ETF
You can also implement a tax-loss harvesting strategy by selling one ETF that has a loss and replacing it with a similar but not not substantially identical ETF or Mutual Fund.
For example, if you have a loss in an S&P 500 ETF, you could harvest the loss and replace that position with an ETF that tracks a different index - such as Russell 1000. If you have a loss in a segment ETF, you can harvest the loss and purchase another segment focused ETF.