That's a great point. I programmed extensively in Excel for real world users, and you can NEVER anticipate how users will make entries. I had one that entered a "space" when he had no data to enter, but a "space" IS data to Excel. Took forever to see it, because... you can't see a space.It's very easy for one tiny thing (like that missed decimal point) to make the simulation very unreliable.
Anyway, if the course was on Excel I'd recommend using all its built in tools for verifying entries (must be numerical, within a certain range, etc.). But I think you can just make sure the documentation spells out the limitations of the simulation. For instance, don't let your 555 simulation run at 10 GHz.
Understanding the limitations of a model is one of the very key concepts of modeling, so hopefully your instructor will be impressed if you demonstrate your understanding of this key concept.