I'm reading The Lean Startup book (you should read it as well) and the author suggests that you should release your web as soon as possible despite it's not complete. This way maybe you engage some early adopters. After releasing something you can watch if your assumptions were right and adapt accordingly.
When reading this it came to my mind an old post by Jakob Nielsen (a usability guru) dated from 2000: The Mud-Throwing Theory of Usability. Moreover, I remember watching him in London explaining it theatrically. His theory suggested that some people were building sites like throwing mud at the wall and see if it sticks.
He, in 2000, advocated for some usability practices, just a minimum (paper-prototyping and testing with few users), before releasing anything. Does it contradict The Lean Startup ideas? Not at all, I would say.
In 2000 usability and user experience weren't as known as they are now and Jakob Nielsen was just making them widespread (and selling his services). Currently developers and designers have assumed the importance of users. When Eric Ries (the author of The Lean Startup) suggests to release as soon as possible, he talks about the MVP, Minimum Viable Product, and without usability tests it could not be "viable".
How much to invest in those tests would depend who you ask. Keep it Lean.