Hi Alyssa
Whether it saves time probably depends a bit on whether you're building something for someone for the first time, or whether it's part of an ongoing relationship with the commissioners of the work. I.e. you get a good feel for each other's expectations as the relationship matures. The size of the piece of work might also sway me as to whether a storyboard or prototype was going to be more useful.
Generally I'd prototype for commissioner agreement on wider functionality (e.g. look and feel/navigational metaphor, particularly on early projects with the commissioner) or to win the commissioners' hearts, then storyboard for the content in a format that the SMEs can easily amend or comment on right there in the storyboard. If there was a particularly complex screen, i.e. one with a lot of complex interactivity, I might prototype that to:
1. Prove to myself that it can be done, or at least give myself more time to come up with a good way to achieve it, and
2. Demonstrate to the commissioner what I'm talking about (assume that the commissioner of the work has no imagination; most storyboards won't do a complex screen justice - a storyboard 'tells', a prototype 'shows'.)
I think there's no definitive answer as to whether storyboarding, prototyping or just jumping in is the best approach. It'll depend on some of the factors above.