Zapster, your post reminded me that I use a Sharpie, too, except that I identify each cable by writing inside the box itself. I write on the box bottom for cables entering from below, and on the box back for cables entering from above.
I = feed In
O = feed Out
3 = travelers
L = Load out
Since I make all connections and strip and hook device wires during rough, there's little chance of mixing them up, but I do it anyway. Plus, the only tool I need at trim is my electric screwdriver, and the freshly-painted walls stay cleaner.
