This is a repost of a piece I did on Android Central. I hope it helps you.
There are several ways to back up your phone. Clockwork and other Recoveries offer you the ability to make a Nandroid of your phone. This is a critical backup procedure and should be used early and often when flashing ROM’s. Other backup methods involve apps form the Android Market like Titanium Backup, My Backup Root and others.
Nandroid backups are full system images of your device at the time the backup was made. Nandroids are an all or nothing recovery tool. They are not good for restoring apps to a new ROM.
To backup data between ROMS there are several good backup programs you can use, and there is no harm in backing up everything on your phone with Titanium or any other backup tool. It’s the restore where people get into trouble.
Never Never Never restore system data from Titanium or any other backup program when moving to a new ROM. The system data from your old ROM will interfere with the new ROM every time.
So after backing everything up what should be restored?
Pictures/Video – Pictures and video on many devices are stored by default to the phone’s internal emcc data partition. Before flashing any ROM’s you want to move these to your SD–Card for safe keeping. There is no need to restore them because the Gallery apps will find them on the SD-Card just fine.
email - Gmail is saved on the Google Servers so no need to back that up. POP/IMAP and Exchange email is also backed up on the mail server so again no need to back that up or restore it.
Contacts - Gmail contacts are automatically backed up to Google’s servers. Exchange contacts are also backed up on the server. Phone contacts can be backed up using the export/import function in the phone app. You restore phone contacts using the same export/import function in the phone app.
Calendar - if you use the Gmail Calendar, Google’s Server automatically backs up your data. If you use Outlook to sync your Calendar with HTC Sync, then flashing to another ROM may or may not support HTC Sync so you should use the Gmail Outlook Calendar sync utility to sync your Outlook Calendar to Gmail. If you use Exchange then the Calendar will be automatically backed up on the Exchange server.
Bookmarks – Many of the backup utilities will back up bookmarks and this is one of the very few pieces of system data that can be restored on any phone.
Apps - What apps really need to be backed up/restored? Apps that hold state or current progress information and that’s it. Apps like Angry Birds (as an example) store your progress locally and if you do not back it up you will lose your progress, so game data can be backed up and restored.
Bank apps? Nope – everything is on the bank’s server. Just re-download and login.
Weather Channel? Nope – the handful of locations you can monitor can easily be re-inputted after a fresh install.
Beautiful Widgets? Nope – If you have the paid version, you can re-download from the market and re-set up your preferences. If you want to save the data so you don’t have to re-download skins then this is another where you can restore the data after re-installing the app.
The big advantage to backing up all your apps with Titanium is that you have a list of what you had installed. You are better off doing clean installs from the market for apps that do not hold state. Another way to manage which apps you had installed is to use AppBrain, it will sync what apps you have installed but AppBrain does not save data.
SMS - Now this one is a personal thing. I don’t care if I lose my SMS. I don’t hold every text message as if it were a precious illumination by DaVinci, however I know that some people do. There are apps in the market, like SMS Backup and Restore and SMS Backup+ for example, that will restore SMS messages, even when moving between ROM‘s but they are not a cure all. If problems develop after restoring, then you want to roll back that restore.
Other than that, what else needs to be restored from an old ROM? Any config’s or screen setups would have to done over again anyway. You would want to take advantage of any features the new ROM has so restoring old setups will do more harm than good.
So, after a very long-winded dissertation, backup apps are good for moving from ROM to ROM, you just need to be knowledgeable about what is safe to restore and what is not safe to restore. If you want to sloganize this in very simple terms; backup everything… restore selectively.