The Fundamental Principles of the WTO

In the context of the plain packaging dispute at the WTO, Dominican Republic's Ambassador to the WTO,  Luis Manuel Piantini, said this: "One of the fundamental principles of the WTO is that Members should regulate in the least-restrictive way."

Is this one of the fundamental principles of the WTO?  Should it be?

Over the next year or so, I'm going to write something about this issue, at least indirectly, and I'm curious to see what people think of it.