I went over many different ways of dealing with these those fixture straps with the author of Electrical Code Simplified. I had the same idea as you yummy mummy - since the box was already grounded and was metal, I could simply mount the strap to the metal box and wrap the fixture ground to the green screw. I was told this more often than not fails inspection because alot of times the metal box is not flush with the surface(eg. drywall) causing a poor ground connection with the fixture.
I was told three methods that would work the best as of 2008 code(these all assume you have left the ground wires long enough to work with):
1) Assuming you have two cables with two ground wires entering the box, wire nut these two together and connect one of the ground wires to one ground screw in the box. Run the fixture ground to the second ground screw in the box and disregard the green screw on the strap.
2) Assuming once again you have two cables with two ground wires entering the box, wire nut these two together along with the fixture ground. Run a pigtail from this wire nut bundle to the green screw. Connect one of the solid ground wires to one ground screw in the box.
3) Assuming once again you have two cables with two ground wires entering the box, connect each ground wire to its own ground screw in the box. Wire nut them together along with the fixture ground. I use this method the most, especially on boxes with only one cable entering.
The whole point is that you want to be able to remove the fixture without disturbing the grounding portion of the circuit in the event that you remove the fixture live.
There are many other methods that will work just fine but may not be code compliant. A forth option is to use a self threading screw I believe(not self-tapping) that needs to meet x amount of thread cuts and you could install your own third box ground screw for the fixture this way.
Sorry I've confused you even more, but there are quite a few alternatives out there.