Well you would have to define what "critical" means to you, however, I would at least hit these slices:
Without specifics, your question is difficult to answer. There has to be some type of criteria to dictate exactly what you need to do (security policy for example). Questions like the following need to be answered before randomly securing boxes:
1) Can this box be down long enough for a rebuild?
2) Do I have the talent to get the box restored should it fail?
3) Where are my critical data/system files located?
4) What exposure does this box pose should it become compromised?
Hope this helps you.
Our scars have the power to remind us that our past was real. -- Hannibal Lecter.
Talent is God given. Be humble. Fame is man-given. Be grateful. Conceit is self-given. Be careful. -- John Wooden