As a regular user I can't edit my cron settings. I get a "permission denied" for everything: listing the cron commands or editing them.
Example:
cron -l
cron: can't open or create /var/run/crond.pid: Permission denied
crontab -e
/var/spool/cron/crontabs/skerit: Permission denied
These are the permissions of /var/run/crond.pid:
-rwxr-Sr-- 1 root root 5 2011-05-27 12:44 crond.pid
---------- 1 root root 0 2011-03-23 21:13 crond.reboot
And the permissions of /var/spool/cron/:
drwxr-sr-x 5 root root 4,0K 2009-12-23 23:01 cron
All users are also added to the /etc/groups file:
crontab:x:102:skerit,www-data