Army Composition scoring in Warhammer 40k is hotly debated and contested topic that you see pop up pretty frequently. Comp in Warhammer Fantasy is very common and pretty much expected given the disparity of the power level in armies, but it is a little bit more rare in 40k. With Fantasy you will usually have a numerical points checklist that will give you bonuses or penalties depending on what you choose for your army. These can be fairly simple, or horrendously complex (see the Swiss method.) There hasn't been a real solid and widespread comp system pop up for 40k. You will usually see different ways of handling it between different events.
In 40k, the numerical point method usually doesn't pop up too often. People who are going to build a hard list find a way to min-max the scoring system to play to their advantage, so this just isn't a very good solution.
Having players judge comp after they play each other opens up a whole other can of worms. What you will run into is a win-tax. Players who get badly beaten will often mark their opponents down regardless of how their army composition looks. This also doesn't really solve the problem.
Completely omitting the comp score is another issue. This can have mixed results as well. You end up with the uber-competitive types bringing really hard lists, and the newer players who can only bring what they own just get annihilated by these stronger players. This usually sorts itself out after a round or two, but in smaller sized tournaments this might be all you are playing.
I think it boils down to knowing whats going on beforehand. If you know there are going to be some hard lists, you can plan accordingly. Something else that can help would be a tracked tournament, this would work well for larger venues. After the first day you split your crowd up and actually have two tournaments going. This way you can separate out the 'ard players from the newer or fluffier players and have the fields be more even. Almost like having prelim or seeding rounds.
One way of judging comp that I have been toying around with would be doing it anonymously online before a tournament. You would have a cut-off date for army lists to be sent in, and you then publish them all online (without player names) and let the tournament attendees vote before time. This way everyone has a voice, you aren't taxing anyone because they beat you. What do you guys think? Do we even need comp in 40k? What is your favorite way to sort out the 'ard players from the soft? Is my idea crap? Let us know in the comments.