Depending on how representative designers are of the target audience, a project might need more or less user testing. Still, usability concerns never go away completely.

One of usability's main laws is that "designers are not users". The usability discipline exists because systematic methods can overcome the gap between the design team and its target audience. You don't have to rely on your best guesses: you can find out how actual customers behave and what you can do to make them buy more.

Read the whole article about bridging the designer - user gap.