No Worries!
If you do forget to publish, that cycle's free magazine will remain as a magazine credit on your account. You can always see the number of magazine credits you have available within your App's settings menu.
That magazine credit can be used for up to 12 months, as long as you maintain an active account. The app will allow you to publish those magazine credits until they are used up, and will take them into consideration at checkout.
If you do cancel your membership, magazine credits will convert to a dollar amount that can be used to publish going forward at a non-member rate.