When must you use IMAO?

You never HAVE to use it. It is appropriate when you are stating a personal opinion about something that others may not agree with, or that you can't really back up with facts. To a certain extent it is acknowledging that the opinion is definitely biased and will come across as arrogant.

Examples:

• IMAO beer is better than wine.
• IMAO Selena Gomez is cuter than Miley Cyrus
• IMAO Texas A&M is a better university than the t.u.

There may be some facts that could support those view points, but the reality is that it is a matter of personal preference and opinions are going to vary. And most people tend to use IMHO instead.