I'm a little confused, but that may be caused by my grasp of the English language. What do you mean by "how many of you let your dive buddies know post dive, you may go out of air on this dive?"
When it comes to drills, I don't like totally unplanned drills. Unless you've drilled a lot and know your buddy very well, it may lead to a sticky situation because your buddy freaks out a little, believing it's a real emergency. If it's your regular buddy, and both are used to doing unplanned drills, it's probably OK. I and my sons have adopted a sign to signify that one of us wants to run a drill: Index finger "drills" into the palm of the other hand. That can be done at the spur of the moment, without having it discussed in the pre-dive chat/planning. The other diver then knows that when something is coming, it's not a real emergency, but he doesn't know what's coming. When we're done, we give the "drill" sign again, followed by crossed forearms ("X") held up.
And unless there's been an incident beyond what normal diligence plans for, there's no excuse for really running out of gas.