If you have NIS configured, it'll be used by anything that needs to map a uid or gid number to a name, or anything that needs a list of groups for users, among other things. That means (IIRC) that having a crontab implies a NIS lookup, because creating a new session for your user needs to fetch supplementary groups from NIS (even for root). 'ls -l' might also do lookups to resolve the uid/gid of your local files. "local files" does not imply that there is no need for YP.