Pretty much how I do it, 18 is good enough, especially if there are important feats. I'm not going to delay vital feats for a concept just to get to an 18 or 20, though. Feats like Sharpshooter or Great Weapon Master are too important to delay.16 is my minimum for starting with a primary stat. I will try to bump it up ASAP. 18 is ideal. I'm fine with letting it sit at 18 for a long time though, since once you get to 20 you're done.
In the context of Fighters and Monks (two of my favorite classes) the flat damage gained from a high score at low levels will almost completely overshadow the rolled damage, and provide a leg up for the many rolls you'll have to make since you only do one thing.
"Mechanically Is always better" has to have some context to be meanongful.Feats are fun, feats are what make your character different from another.
But, bumping primary stat mechanicaly is always better than feats.
Those should have been different resource pools.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.