In a way, this is really a tax question.
Someone who has stocks held over a year (366 days) and makes under $39,376 in 2019 should definitely sell from taxable until their income + long-term gains add up to $39,376. The bottom long-term gains tax bracket is zero. You pay nothing.
The next bracket, 15%, extends all the way to $434,550. This call is harder to make. Will tax rates be above 15% before you pull money out in retirement? It might make sense to have some "tax diversification" and do both: sell some taxable, convert some in retirement accounts.