While I concur that no one is absolutely thier alignment, I nonetheless have to agree completely with Henry.
As a general rule, it is pure nuetrals who are motivated by convienance. To suggest that you are not chaotic in your behavior because the consequences would be inconveinent is to suggest that you are neutral and not chaotic. If a chaotic doesn't do rebelious and defiant things even though he knows it would be unwise, then probably he isn't strongly chaotic or even chaotic at all. A true chaotic wouldn't think about things; he would simply respond with his viceral emotional reaction - nor could he conform for long without exploding.
The basic test to me is to always compare such alignment tests with similar (and less contriversial) alignment tests for Good. If a neutral good character fails to show compassion because it is inconveinent (he might not be able to buy that +3 sword), or fails to assist the helpless because it might draw unwanted attention to himself, or fails to stand up for his beliefs because it might be dangerous - no one is going to allow him to claim goodness for long. A character might be allowed to weasel around one test or the other by claiming some higher purpose or other special defence, but eventually one notes that the character is all the time trying to evade the strictures of his alignment and therefore must not believe in them that strongly. For such people, I always recommend pure neutrality.
Alignment tests which we make for one alignment, must have equivalent alignment tests in every other alignment, else we are ourselves showing preference for an alignment. The various people who make claims of superiority (X alignment is enabling) are saying more about thier own alignment views than they are about the alignment system. I don't find evil to be enabling at all, instead I find it enslaving and indeed mentally disfiguring. Nor do I find either chaos or law enabling. Tell me how much free will a serial killer or a pedaphile or a heroin adict has left? Wanna guess what alignment I believe most closely conforms with my own beliefs?
At their hearts, each alignment system causes a person to stop doing certain things because they lose the desire to do them.
Melkor may wish to think that he is a rebel, as our chaotic leaning post 1968 society glorifies the deviant, the avantgard, and the disident - but there is not alot of evidence of Melkors chaoticness from the limited information we recieved.
And there is even less evidence that a character in an RPG with Melkors mode of behavior could claim they were Chaotic. At least we have Melkor's testimony that he internally feels rebellious and mentally struggles with the confines that society put around him. He might possibly be slightly chaotic in nature. But in most RPG's we can't spend alot of time quantifying mental anguish (we could but it would mean an entirely different sort of game), and we can't rely on the players claims that his player is experiencing emotion because it is a VERY VERY different thing to have your reason clouded by emotion, and to say that in abstraction your character's reason is clouded by emotion while all the while maintaining your emotional distance. For RPG's, especially those of the heroic mold, we have to rely on the characters actions to display thier internal states. A good RPer who is in touch with his characters emotional state tries to bring this out strongly in his character's actions - even if this percludes playing the game like a tactical wargame.
And it is hardly convincing to make the claim that the character is motivated to restrain his natural inclination to law or chaos because he is a coward and fears the consequences, if he is also the sort of person who (as most adventurers do) seeks out danger and adventure. Not that it couldn't be done, but it would require playing the character far differently than most people who shrug off thier characters philosophical outlook play their characters.