Like many of the others that have responded to this thread, I tend to want to storyboard right in the tool and get started with the big picture. It helps me to build a screen when I find just the right image or graphic that expresses the thought or concept I'm trying to portray on the screen. This normally helps with my process to produce a narration and on screen text or interaction. The problem with skipping the storyboarding phase is that you can get caught in the trap of entering into "the zone." "The Zone" is where you put effort into putting something together like editing the graphic, maybe adding animations, or interactions because you are in the tool and you think, "this is just what I want." However you forget that the storyboard is supposed to be building and end product which should be what your customer wants and gives them a chance to read/interpret how you've understood their content and what you think may be the best way to present the information. The trick is finding a storyboarding format that gives the reviewer enough information to really know what the screen will look like and how it will act, that contains all the elements that your team may need to be able to produce all the different pieces after the storyboard is approved, and that can accept comments in a logical way that whoever is responsible to make edits can go back and do them as quickly and easily as possible.
I do like the idea of a prototype for look and feel for anyone reviewing a storyboard. A small portion of any course should be functional so that when something is described in the storyboard the reviewer has a visual image as a reference point.