Hello and welcome to television.
My copy of SMPTE RP-27.3 "Title and Action Safe" document dates back to 1989 and that one was an update to the one in 1983. I joined NBC in 1976 and it was already an old standard then.
Sorry. It's not a standard. It's a "Recommended Practice."
Periodicaly, someone will stand up and claim you don't need the two safes any more with all the flat screens and projectors out there. That lasts until they see the motel room with the fifteen year old TV on the dresser. Oh, and the modern, up to date plasma flat panel in our conference room is set to cut off the top and bottom of the picture.
If anything, the flat screens made the problem vastly worse. You used to be able to ignore the extreme edges of the picture secure in the knowledge that few if anybody was going to see them. Not any more. Computers show you all the theatrical bits right out the edges, so you *can't ignore them*.
You are more or less forced to conform to the two safe areas (80% and 90%) while still protecting the full screen. Periodically, a forum poster wonders if they can jocky the visible parts around to look good on their tube-type monitor. Sure, as long as you don't leave vast black areas missing around the edges.
Having said all that, the document is a recommended practice, not a standard. It's up to you completely whether or not you want to risk chopping off the work. The other thing you can do is force a viewing situation. If you know for an absolute fact that your work will be shown only once and on a certain projector, you can tune your whole world for that one show.
If, however, you intend on general distribution, there is only one way to be safe.
Koz