Yes, a Nandroid backup of a good, known setup is your best safety net. Just make sure you've also got it backed-up somewhere externally to your phone just in case. If you run into issues, you should just be able to reboot back into ClockworkMod and restore that known good backup.
You've also got the ability to flash everything completely back to stock if necessary, too.
As far as ClockworkMod not "sticking", have you seen the following in this post?:
http://androidforums.com/verizon-ga...gs-root-samsung-galaxy-nexus.html#post3736600
Flashed custom recovery won't "stick"?
Do this:
- remount
/system in read/write mode (use something like the
Root Explorer app or
ES File Explorer; both have root modes (check the app's settings) for remounting
/system as writable)
- rename the
/system/recovery-from-boot.p to
/system/recovery-from-boot.p-bak
(note: you could delete it, but renaming is much safer since its easily reversible)
- you could also do this manually in an adb shell or on-phone Terminal session:
+
mv /system/recovery-from-boot.p /system/recovery-from-boot.p-bak
Cheers!