It's all perspective.
A blue belt showing a white belt the basic armbar from the mount is okay. That same white belt turning around and showing another white belt that same move might also be okay.
Many times in class someone jumps in late and didn't get to see the formal instruction by the black belt. So the white belt has to figure out what is going on somehow. Another white belt isn't going to be able to go into all the details, but a rough understanding is okay. Hopefully the black belt will have enough time to stop by and correct any mistakes and possibly demonstrate it again. But if not - no big deal. The white belt has an idea and it will be modified or corrected in a future class.
What would be a big NO-NO is what I saw happening at my old school:
Some TMA instructor would take a private lesson or two from my instructor, then go back to his school across town and teach the move. The problem with this is that the guy in question didn't do regular classes, didn't spar, and was spanked badly the few times he did have to roll with other white belts from the regular class. This guy going off to his own school and trying to teach without any sort of support, either from higher ranks or the black belt instructor himself is doing the students a disservice as he is presenting himself as the sole authority in that environment.
For example I have no trouble as a purple belt showing and teaching people things. But I have back up support. There are other purples, browns, and blacks that are readily availible to correct any mistake I might make, or answer any questions that we as a group may have. I am not the sole authority.