IT depends on the circumstance. I think that I would file the form, even if I was only finding out through conversation. Conversations held with customers let you in on a great deal about their situations. It is one method of helping find out WHO your customer is and one method of assisting in monitoring the customer activity.
Now, there are exceptions listed on the back of the #4790 form. Perhaps your customer fits into the exceptions profile.
The views expressed, herein, are not necessarily those of my employer