Is it premature optimization to add database indices?

A colleague of mine today suggested that we go trough all queries in our application and to add indices accordingly.

I feel this is premature optimisation because our application is not even released yet. I suggested to monitor for slow queries once we go live and then add indices accordingly.

What is the general consensus when designing your database, should you add a matching index every time you write a new query? Or is it better to just monitor and see how it goes?