I'm not primarily indexing, but investing in individual stocks. So, I sell when there are good opportunities to sell, but try to keep 6 months-1 year in cash for withdrawal.
If you are indexing, then a regular schedule can be a good thing. It is the same dollar cost averaging logic on the way out, as the way in. It will not optimize your withdrawals, but you won't get caught at the worst possible moment, either. On autopilot, you won't have to think about it so much, which is part of the point of indexing, after all.
One point to be careful about: doing this automatically, or in the year of withdrawals themselves, could lead to unintended consequences with your income planning for ACA purposes. Income, taxes, and ACA are aspects of the same planning and decision making process, for me.