I noticed at http://arduino.cc/en/Reference/StepperBipolarCircuit
... heading Circuit for Bipolar Stepper Motor, npn transistors are used as NOT gates (inverters) to reduce the two lines coming in to one side of the h bridge to only one. This energizes the windings of that side of the bridge with logic signals of 0 and 1, respectively, so neither polarity is ON at the same time. It should be noted that the transistors can be replaced by logic gate inverters (NOT gates). The only inverters I have, come six to a chip (HEX inverters) which just happens to coincide with the number needed for a three axis cnc machine that uses bi-polar steppers, so mighten it be helpful to make note of this on that webpage? It keeps 4 resistors per motor in your 'to be used' bin, not to mention the transistors, and mostly eliminates worry about current issues.
If I'm wrong on this, please clarify.