As Bob pointed out, it is very subjective. Like the part where they use the colored object and say "tell me when you can tell it's red" (or whatever color). If I'm a little hot, it takes longer to tell, if I've been waiting for an hour in the cool room, I can pick it up right away. Same guy, same eyes, very different outcomes.
Good point, between the subjectivity on the part of the examiner AND the part of the person being examined, EDSS assessment, at best, is just a rough guide.
For the most part I think it's generally accepted as the rough guide that it is and I think it's generally understood that it's not perfect and is only the best we can do right now. On the other hand, as Tim pointed out, sometimes EDSS might be taken too literally and half a point difference either way can keep you out of a study.
I'm also like Heather in that I always try and "do well" on the walk and the other stuff.
I think an accurate EDSS is to the benefit of the patient and doing your absolute best is the only control a patient has in reducing subjectivity.
At some point in the future I imagine researchers will implant electodes into the axons of MS patients and run the wires to an external guage which could be hidden above the hairline of the patient. That will be handy because at that point we can brush a few hairs out of the way and say "Lew's got good conductivity today, he's my golfing partner" or conversely
"Mikey, you be Lew's partner today!"