> Now, I could extend my user-global ignore

I'd add .nvmrc and .npmrc if you work with NodeJS.

but if I add that to my user-global config instead of the projects I'm working with, I'd be making the deliberate choice of fixing things only for me and not anyone else for pretty much the exact same cost.

I think rules for your personal tools, like editor-specific ignores belong to your user-level config, but anything around the project's tools and artifacts belongs in the project's gitignore.