Make a backup 1st.
When you restore a backup, it will install in exactly the same position as when you made the backup. If you delete your data before you make a backup, then you will have no data when you restore.
The process you should follow when installing a new rom is to make a backup of the current one on your phone. That way, if you dont like the new rom, you can restore your backup and you are back in the exact same position before you started installing the new rom.
Once all of your apps restore and you think you like the new rom, make a backup of this rom. This will be a restore point for you as once you start loading mods and such you want a restore point to go back to in case something doesnt work well. Backups are your friend, use them and they can save you tremendous amount of time and angst.