Sure. Lots of factors go into deciding a data store, and you're right that you shouldn't just choose at random.

I disagree with the implication that if you do actually think about it, you'll necessarily land on MySQL or Postgres.

We landed on Mongo, and one of the reasons we did was because we were uncertain about the product direction. We knew we were going to be making massive changes to the schema. We didn't want to (couldn't) spend the time up front the think through the precise implications of every table in the system. In a very real way, we didn't even know what data we were going to be storing!

So, we mostly skipped it. We came up with a simple set of documents, our best guess, and ran with that. We made mistakes (we knew we would), and we corrected them easily. We saved time up front, and really haven't regretted it at all.