Inside FCP is always best, or use Final Cut Library Manager from ArtcicWhitness. Even though I'm very comfortable with poking around in the Library package contents, I avoid it as much as possible. Accidents happen, and an accident inside that package could be disastrous. www.arcticwhiteness.com/finalcutlibrarymanager/
Wondering wrote: Was making space and wondered if deleting render files from the finder (Library>show package) would be harmful vs. usual way from inside of FCP. Thanks.
What Ben said is correct. However if you use the library inspector Storage Locations>Modify Settings>Cache and designate an external folder to store the cache files, those are easier and safer to delete in Finder.
That is because they are all within a single bundle -- separate from the library. The cache bundle includes thumbnails, waveforms, optical flow analysis files and render cache. The entire bundle can be deleted. I normally do that when FCP is shut down.
But (as Ben said) the best solution is using Final Cut Library Manager -- especially if you have many libraries. Instead of treating each one separately, it scans them all and allows taking collective action on all.