In some cases, I would say that you should definitely preview properties prior to taking potential buyers. In other cases, I would say that it's not necessary. I typically will preview properties for out-of-town buyers so that I can provide them with more information about the property. This will allow them to eliminate those properties that do not meet their needs. And when they are in town, we can use our time much more efficiently. There are instances when I will preview properties for local buyers. This usually is done for those who are short on availability time due to work or other commitments.
Once you are practicing, you will develop your own standards as to when to preview or not.