Many advocates of the lean start up method encourage you to take little bets by shipping a product as soon as possible or at least opening it up for potential customers, regardless of finalized (or any) design. From there you’re encouraged to tweak and make iterations based on your learnings. But design and user experience clearly have an impact on the way people will appreciate (or not) your product.
From the start, design should be considered an integral part of the product and thus should be an ongoing piece of the conversation– not something slapped on at the end. I like to point to the example of the iPod. Steve Jobs obsessed not only over the design of the product, but even its packaging down to the smallest detail (so much so that he has a patent out for the packaging of the Nano), before letting it go out of the door. Sure there were other portable music players, but the iPod was able to beat them out with its designed intuitiveness (and perhaps a massive advertising budget played into it as well).
The point is though, that people do judge a book by its cover. Design is the essence of what makes some companies and their products great. So as long as you understand what your company’s strengths and core values are, little tests might be OK for you– just make sure it’s the right kind of “little” test.