How rough should a prototype be?

Too polished, and people critique the colors instead of the concept. Too rough, and they cannot imagine actually using it. Fidelity is not a quality setting to maximize, it is a variable to match to the question being asked.

We map prototype fidelity to that question directly: paper flows and rough sketches for testing whether a concept makes sense at all, clickable wireframes for testing a flow, and coded prototypes for anything touching pricing, trust, or a purchase decision.

Getting this match wrong in either direction wastes a research session. Getting it right means the feedback you collect is actually about the thing you meant to test.