What is the best place for an Add button in an iOS table view?

I’m trying to figure out where to put the Add button on my table view (on iOS). Its purpose is to create a new row in the table view, either inline or using a modal form.

There are 3 common positions for the + button:

• in the navigation bar (contacts, calendar),
• in the toolbar (reminders, mail, notes)
• as the last row of the table view (reminders).

My (weak) guess is that Apple internal guidelines are:

• in the navigation bar when the addition is not the main purpose of the app (contacts and calendar are mostly opened to look at existing entries),
• in the toolbar when the addition is a very common operation (the purpose of reminders is to add things to do, mail and notes to write).

Is there a correct answer?