Thinking About Prototyping - Paper Prototyping: The Fast and Easy Way to Define and Refine User Interfaces - page 42

context.
How Good Should It Look?
This is a complex question, so I'm providing only a partial answer here and will tackle other parts of the
answer elsewhere in this topic. The short answer is that a prototype has to look (and act) realistic
enough to elicit feedback for the issues you're most worried about, but it doesn't need to be any better
than that. Chapter 12 goes into detail about the kinds of problems that paper prototypes are good and
not so good at finding. Chapter 13 presents evidence that paper prototypes seem to reveal about as
many (and as severe) problems as you'd get by testing the real thing; as you saw in the previous
chapter , there are plenty of real-life examples to back this up.
One drawback of a prototyping tool that gives you several options for specifying the appearance (fonts,
colors, shading, and so on) is that it's easy to get caught up in tweaking the design to make it look nice.
If you end up changing it later (and has there ever been a design that didn't get changed?) then your
tweaking time will have been wasted. A sketched prototype helps you avoid the temptation to perfect
the appearance because it isn't supposed to look finished. Time is money, so you want to weigh
whatever effort you're putting into a prototype against the benefit you'll get out of it.