"Should monetization be an after-thought?" The answer depends on you - is your primary goal with this application to make money? If so, then no, monetization should NOT be an after-thought.

I would spend some serious time thinking about ways to monetize your app. While everyone seems to jump on the bandwagon of advertising for money, many are coming to realize that it's a huge gamble.

Rethinking, revisiting and re-evaluating your idea isn't such a bad thing. I would follow Sachin Rekhi's advice - is your product something that people will pay for? If not, I would put some time into thinking about whether you can mold your idea product into one that people WILL pay for.

Even if it's something as low as $5 per month, you can make up the difference in volume (if your product is good enough for MANY to like). Your main goal should be to break even as fast as possible and the subscription model can allow that.

Or better yet, split test the two - advertising vs subscriptions. Either way, I wouldn't just push this to the back of my mind because it seems like you can't find a clear answer. That's only more reason to think about it harder. Take some time to research and analyse apps that are successful in terms of charging subscriptions. Think about WHY people are willing to pay for them.