I usually end up knowing what job and personality I want and then make the Mii around that job and personality. Sometimes I pick them strategically, but when I've already made the Mii I pick it based on the Mii so I pick that, since when I do pick strategically it's before I make the Mii.
I usually create OCs especially for Miitopia so it works.
(Example- in my recent playthrough, I theorised that Cautious and Tank were a bread and butter combo, so I used it, and made a mii that was designed to fit the Cautious Tank role.)