Clearly, spreadsheets are subject to the GIGO principle - garbage in, garbage out. If you assume an annual return of 10%+ for the next 40 years, you'll most likely fail to achieve it.

Having said that, as long as you update your spreadsheet regularly, each time see how real life diverged from your projections, modify to account for that divergence, and see if it becomes a regular pattern of being overly cautious or overly optimistic, you're much better off wet the spreadsheet than without.