Perhaps the first, and most pertinent driving principle of interaction design is that the prototype serves as the record of truth for the development team, as a vital stepping stone towards accurate implementation of the intended behaviors. The prototype also provides more visceral, accurate, assessable information for evaluating the design’s utility and enabling non-designers to buy-in, stakeholders to sign-off, and customers/users (preferably alpha users) to effectively judge. And the prototype, in demonstrating both visual as well as behavioral qualities, can provide a more “well-rounded” sense of what solution is supposed to be, helping implementation experts estimate for resources, timing, and methods needed to make the actual shippable product.
So, if the prototype is that critical for the design process, then you’re correct to assume that every step of the design process and every deliverable created along that path should be directed towards getting to the prototype quickly and efficiently. In other words, don’t waste time making a perfect flow diagram or set of wireframes …satisficing (from herb simon, the idea of sufficient yet valuable) is paramount to sustaining forward momentum.