I don't, full stop. If the modifiers are so good that snake eyes is a success, then snake eyes is a success (and for a pass/no pass task, don't even roll 'em). There are few things I hate more than critical failures (or successes).
My character, recently and much simplified: "It is just possible that you are thinking of doing something that, by my allegiance, I absolutely cannot countenance. I'll be in my stateroom; let me know when you've finished not doing what I'm definitely not talking about."
Even if you never play GURPS - or don't know thing one about the game system - their books are great sources for Traveller and Charted Space. Long live His Imperial Majesty Strephon!