We came to this rule after far too many incidents where some sort of mock up (Photoshop, HTML, whatever) was shown and taken as done. Then we got the questions (possibly unhappily) about where it was and when it would be done because obviously ”it must be” since we “showed it”.
The rule served us incredibly well for years. It put a hard stop to all miscommunication. Everyone understood exactly where we were in the project. Either we knew what it was going to look like, or (if it was ready) it was done and awaiting their approval.
Shortly before I left we got a new graphic designer. He wasn’t embedded with the programmers. He didn’t know THE RULE. Sure enough, we were asked why a design we’d never seen before wasn’t ready yet. Because he made a mock up in Photoshop. He told them it was a mock up. Doesn’t matter.
We basically follow THE RULE at my current job. It still works wonders.