Reply to iZotope Ozone 4. When, where and how much?

When do you use the iZotope Ozone plugin? While you are engineering a track or when you've completed it? Do you usually throw it on the master channel? What preset do you use and what loudness amplitude?

Sorry for all the questions. Been using this for a while but curious to see if I'm doing it wrong. A friend told me I should never have it on my master WHILE producing cause it messes you up.