Despite the handiness of Cronnix and other utils, I find that several factors promote the editing of crontab files for use in corporate settings:
# /etc/crontab
SHELL=/bin/sh
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin
HOME=/var/log
#
#minute hour mday month wday who command
#
#*/5 * * * * root /usr/libexec/atrun
#
# Run daily/weekly/monthly jobs.
# Modified by YOURNAMEHERE for XCORP, DATE.
# Adds weekly Repair Permissions plus reschedules periodic jobs
# for times when systems are running and awake.
# Adds weekly prefs scan, -s reports only corrupt prefs.
15 17 * * * root periodic daily
30 9 * * 2 root periodic weekly
30 10 1 * * root periodic monthly
15 18 * * 3 root diskutil repairPermissions /
15 16 * * 4 root plutil -s ~/Library/Preferences/*.plist
30 16 * * 4 root plutil -s /Library/Preferences/*.plist
[robg adds: I haven't tested this one, but I do understand the business perspective -- untested external tools aren't particularly welcome at my office, either! If you try this, you'll want to use tabs between the items in the crontab list; they're spaces here to make it a bit narrower.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200502082215230