Do all airlines do this?
No, not all. But all OneWorld, Star Alliance, and SkyTeam airlines offer this service. Check with your airline before booking.
So, am I guaranteed first-off with this special tag?
Almost. Obviously, baggage handlers, just like all of us, sometimes make mistakes at work. And these mistakes occasionally include not priority tagging your bag and/or putting it on the conveyor first. So, no, you're not guaranteed, but it's rare that you won't. The key is to not go all Alec Baldwin when your suitcase isn’t first off the belt, or it may mysteriously end up in Iceland the next time you fly.