RunningBum - you are correct, and thank you for helping me to understand (no need to apologize). I thought I could only choose First In First Out (FIFO), or Average Cost, but I see now that there's an option to use Specific Identification (SpecID). If you use SpecID, then it doesn't really matter whether you reinvest or not, as long as you pay close attention when you sell. I'm here to learn, as well as to try to help others, so I do appreciate it when people call me out for saying something wrong!
P.S. I just found this, which may help others using SpecID: "If your taxable income including capital gains is below (for 2019) $78,750 if married filing jointly, long-term capital gains have a 0% tax rate. So you may want to sell the shares with the lowest cost basis. That is, you may want to “harvest” the largest gain possible." My plan, prior to RMD age, is to sell as many shares as possible with the lowest cost basis, while keeping MFJ to <$78.75K, so that my future tax liability is lower.