It depends on what a startup values, as to whether or not they should create product principles.
If a startup values creating a repeatable, predictable product and engineering culture, they should create them.
In this episode Des Traynor and Paul Adams discuss the principles upon which we've built our product, and indeed our business, here at Intercom. They talk about the value of having a clearly defined set of principles and how they can become the foundation for cohesive decision making.