However, just be aware, that at least in the past — maybe not after Gutenberg’s release — everytime you update WordPress, it will put the twentyseventeen folder back.

I suspect, that after the Gutenberg release, you’ll start seeing twentynineteen instead of twentyseventeen, by default.

If the theme you are running ever has an update then you need to think about any customizations you might have done. Those customizations should have been done in a Child theme but if they weren’t done that way, it might be best to wait and create a child theme first. Move your customizations over to the child then do the update.

And I’d make a backup copy of the base theme before I did the update rather than chance losing my hard work by forgetting something.

You can delete any theme you aren’t running if you are confident you can reinstall them manually if you ever need one of the default themes.

I do the updates just to keep the notifications out of my hair.