Seems like overkill imo. Simply because I usually get ideas while I'm inside the game, not when it's a mock up.
Also, it's easier to edit in game rather than in photoshop.
However, I do know that Shiny (Fundamental UI) did this, and his UIs usually look really good. Here's an example
http://underworldexodus.com/forum/files/12566643763.7.6.jpg
But I think it's only worth doing if you're going to create custom graphics.
