An Introduction to Decision Modeling
Yes, I agree that choice of distribution is often very important to modeling outcomes. If you're not sure which is most appropriate, you can experiment with different ones as you just did to see whether and how they affect the results. By the way, Guesstimate supports more distributions than the three you mentioned, but the others involve a little more work to incorporate into the model. You can even define your own distributions from sample data that you upload.