In The Technical Writer as an Outsider: How Ambitious Are You?, Tom Johnson talks about the appropriate place in software development to get the technical writers involved, and makes some interesting observations on what the mere existence of external help means about a product.
He makes an interesting argument that the best place is at the end of the development cycle. He is right in that this affords the writer the chance to see the product with a perspective similar to that of the actual user, not having been biased by the close association with the development process that blinds every creative person (including writers!) to the non-obvious elements of what they have created.
As a tech writer, I have experienced that bias repeatedly over the years, and it is a hard perspective to overcome. The perspective of the outsider is essential to creating good usability, but only getting that perspective at the end of the development cycle means that the feedback loop is longer than the Enron audit trail or completely non-existent.
What is needed instead is a means of inserting that outside perspective continually throughout the design process. Some amount of it must come from within each designer, but you must also design a mechanism for getting a true outsider’s perspective at each milestone or iteration—ideally from someone who hasn’t seen the previous milestones/iterations. But having an additional perspective at each of these steps from the same experienced tech writer will help you ensure the consistency necessary to create a polished product.