Assuming you're not omniscient you'll be refactoring regardless. The difference is whether you'll be paying as you go (clients want a JSON API, we need to add new columns to a table but it'll lock rows) or if you'll be taking on technical debt to be repaid in the future (turns out Mongo sucks and we would do much better with Cassandra for serious horizontal scaling).

I believe that if you aren't extremely certain about what the future holds it may be best to work with a more flexible technology first and transition to a more structured setup once you have solved for your problems and identified intended future features. And if you are extremely certain about what the future holds you're either insanely good at your job or just insane.