My family shares a near-Terabyte iTunes library on share.
My playlists always use a "base" smartplaylist with stuff I don't like filtered out (unchecked songs, podcasts, ebooks, single star ratings, played in last 90 days etc.).
So, I have major time invested in check / uncheck. I use the COMMAND function, and want to share a caution. Yes, it is very useful. But warning - iTunes has no UNDO with this. (Bad, bad Apple developer... go to room room without dinner.)
If you use COMMAND with ratings, check / uncheck, extensively as I do (so I don't have to listen to songs that my kids like, without actually deleting them from the library), beware the COMMAND- check/uncheck. Do it wrong, you'll lose all the checks or unchecks, or anything else you mass-change with it.
My suggestions... before you mess with COMMAND multi-selection (use them in any combination)
Never, never mess with COMMAND function when viewing your entire music library! Do it against searched lists or playlists ONLY
Make a smart playlist of the unchecked songs, then save that to a 'real' playlist
Backup your library
If you use Time Machine, you can recover the library, but that's a hassle. Just use it on subsets.