Earlier quoted context omitted.
You should peg dev-deps just like any other dep.
I prefer not to add an extra step for newbies devs contributing to my open source projects. So if someone makes a PR and it's not well formatted, not a big deal. For work, sure agree.
They don't need to apply prettier, but it does no harm that it's sitting in their node_modules pegged to the version you're using.
Also, you may be underestimating beginners. If someone can learn git, how to make a substantial code contribution, and lift it into a PR, they can run your `npm run prettier` step. :)