If I understand how it works, NFS security relies in the fact that the clients can be trusted.
I'm worried about some users accessing files from others. Yes,
root_squash option avoid root user on clients to access the files, but if someone has root privileges on any client, he can su into any user and then access the files they want.
I'm currently exporting the files to all the local network. I'm thinking on export only to a netgroup of trusted clients --but it will be probably a pain to maintain, and it only obscures the problem a bit, the problem still persists if some trusted client gains root access.