If there is a "ledge" or "drop off" where the garage door comes down...
We typically grind the ledge very well, and then tape the very corner of the edge with red duct tape. This ensures a straight line at the very start of the epoxy project, which is what our customers see as they are opening the garage door from the outside. This also ensures that we are not applying epoxy to the actual corner edge, creating a weak point that could peel.
HOA's throw a fit when we go past the ledge or any further out. Even if the epoxy is still contained behind the closed garage door, they don't like us applying epoxy there. We find that it is easier to let the homeowner apply a concrete stain that compliments the epoxy color, and **** right up to our epoxy line. This way, not only are we avoiding coating a sharp corner with epoxy; but we also are avoiding any bad things that could happen when leaving a garage door open a few inches with epoxy curing inside.
If there is no "ledge" or "drop off", we lay down duct tape on the inside edge of where the garage door comes down. This usually takes two workers about 15 minutes to carefully lay the tape down, an 1/8th of an inch away from where the door comes down. We also use blue tape to tape off the garage door itself, just in case.
m237b, the cut and fill method seems like a great idea. I am hoping someone else will chime in on this; I would like to see photos of finished projects where this method was used.