As I understand it, it's all based on a baseline, and while theoretically Xanax, Valium, Alcohol, or any sedative/depressant could help minimize or eliminate the stress response to a question + lie pairing, it really wouldn't affect the question + truth pair (i.e. the baseline questions).
If I understand it correctly, the tack is to be used on TRUE questions because it will cause a reference stress "spike" to what should be a non-stressful question + true answer pair. Therefore your "spike" to a question + lie anwer pair will appear the same, and therefore should trigger a negative result to the polygrapher. The stress spike to the true question might, however, clue the polygrapher in to what you're doing.