Mhh for me it is always a combination of the two. I like making a good high level plan, but for certain problems it is better to make the plan as you go, because you can't fully understand the shape of the problem and the potential solutions as you start. Sure maybe that means your first code sucks and is a prototype that should be replaced, but if you are aware of that and don't pretend otherwise I don't see why this wouldn't be okay.