I usually consider my start-ups to be pre-trading/revenue stage, where it can have a url & be in active development, but not be necessarily be an active registered company. Generally I prefer to avoid the expense (and in some countries the hassle) of business registration if I'm not sure the start-up will reach launch or revenue stage.

In my view, a startup is a commercial project -- which these days can easily be done online without formal incorporation, even by a single person. And if having a formal company hurts more than not having it, the choice is simple.