Personally, I'd say always wireframe / sketch before coding.
The reason behind this is the interface is what the users see and use. Your app's design (from a UI / UX perspective) will almost certainly come out better if you spend some time upfront on design.
As a bonus, you'll likely write less code since you'll know exactly what you're building. Without at least a (revised) sketch in place, you may end up unnecessarily generalizing methods / classes, or adding stuff that you end up not needing later.
Anyway, that's just my opinion. I at least do some sketches, with revisions, before starting an app.
The reason behind this is the interface is what the users see and use. Your app's design (from a UI / UX perspective) will almost certainly come out better if you spend some time upfront on design.
As a bonus, you'll likely write less code since you'll know exactly what you're building. Without at least a (revised) sketch in place, you may end up unnecessarily generalizing methods / classes, or adding stuff that you end up not needing later.
Anyway, that's just my opinion. I at least do some sketches, with revisions, before starting an app.