Should you charge for your app? How much to charge for your app? Honestly, we can replace the word app here with services, or products... the answer to the first is yes. The second is where it gets tricky.
Start by asking yourself, Honestly, How much would you pay for that app? Would you subscribe? Is it an app that will be used daily? Hourly? or just every once in a while? The more frequent the usage, the more likely the subscription model. Otherwise, why would I as the end user subscribe? The sub-system has to make sense to the user, not to you.
If you build it right, you won't need to worry about too much maintenance. But you are right, you need the incentive to continue maintaining an app. But again, what about your target audience? is it a niche market? They won't subscribe unless it is highly sought after, something very very important to them.
You get the gist. What value are you providing your end users? That's what should decide the charge amount and style.
As for the app store, the new feature of providing codes for your users can be used to give away a free month. So it's not a trial per se, just a free month, and they get to cancel their subscription.
The guidelines are (i'm paraphrasing): "Be clear about what is charged and what is free and stick with it. As long as that does not change, Apple has no issue." So if you are saying from the beginning this is a subscription app... then they have no issues with giving away a free month or even more. That's an offer. The key with App Store guidelines is Transparency. Don't try to swindle the users. Be upfront about everything, and you will have no issues.
Hope that helps!
