Depending on how you look at it, it may not matter one way or the other. I haven’t ever taken an RMD so take whatever I say with a large grain of salt. First of all, the amount is already determined in December of the previous year, so that won’t change either way. If you look at your investments as a whole, all the taxable, the pre-tax, and the no-tax buckets, and you balance you desired allocation across all of them, then in theory, when you “withdraw” from an IRA, it is effectively simply a transfer from one account (IRA) to another (taxable), and a liability of taxes of course (and taxes are just part of the usual annual expenses). So if you need to sell 100 shares of ABCD in the IRA, and them move that cash to your taxable account, since you liked the position before, you can simply buy 100 shares of ABCD in your taxable account and everything remains the same. But since there are taxes due, and you want to keep enough cash to pay the taxes, you might only be able to buy 75 or 85 shares of ABCD, but the general principle still remains - if you liked the position before, you should like it ten minutes later after the money moves from IRA to taxable. Now, if you didn’t like the position before, then WHY didn’t you sell the ABCD previously? If they allowed RMDs “in kind” (the actual shares or bonds valued on the day of transfer), then I think more people would have this mindset.
Now since the AMOUNT is pre-determined, then the number of shares that must be sold will change throughout the year. But since there is no reliable way to know if the market will be lower, higher, or flat, then you may as well choose randomly. Can do at the beginning of the year, can do at the end, or quarterly (if you don’t mind the slightly additional work). Sometimes, if your stocks have sudden large gains during the year, and you think they are overvalued, you might want to sell [some of] them anyway at that time. But that still doesn’t determine when the actual cash moves from the IRA account to the taxable account.