Assuming you are talking about Zimbra OSS Version:
please follow any of the method which works to your liking from the wiki
Open Source Edition Backup Procedure - Zimbra :: Wiki
* best backup is which you can restore...so when you choose a method please make sure you know its works and it restores you 100%
Yes..taking a COLD BACKUP of everything (given the fact you do it right) in /opt/zimbra folder will give you 100% good backup
Raj