I normally only backup files, Exchange, SQL, and System State. I've never bothered to backup installation directories. You can, but my thinking is always along the lines of possible corruption with the folders and files, and that would be introduced again when they're restored.
I have successfully restored an Exchange server to working order, along with all the file shares. That was restoring Exchange to a B2D folder and then using Exchange disaster recovery to bring it online; restoring the files to the locations created (restored file shares and all permissions), and the System State of the server.
The other thing is space...backing up each server completely would mean using multiple tapes...it's a bit of a grey area. Some people would recommend backing up everything and having an ASR backup of every server...others would say no.
What does your policy dictate?