There's no "urban myth" about it. Cocaine stimulates the CNS, Benzo's depress it. You may think it's a simple case of one cancelling out the other. But anybody that has a clue about neurology would tell you otherwise. It sends mixed signals through your cardiovascular system and can quite easily lead to a serious case of heart arrhythmia.
If you want to smoothen out the effects of a stimulant and minimize it's withdrawal potential, then I would recommend taking 400mgs of Chelated Magnesium, 400mgs of L-Theanine and 8gs of Myo-Inositol about an hour before consumption. The Magnesium will regulate the Calcium Ions entering the Axon Terminal, preventing a cascade. The effects of the stimulant will be less intense, but will be longer lasting with a smooth roll off as opposed to a sudden hard crash. It will also help to prevent the blood vessels from restricting, helping to maintain a healthy heart rhythm and blood pressure. Theanine acts as a Glutamate Antagonist and can greatly reduce physical side effects from stimulant use, such as jaw clenching, grinding and muscle twitching/spasms. Inositol acts as a secondary messenger between the neurotransmitters allowing them to function more efficiently with the corresponding receptors. There is also some evidence to suggest that they help to upregulate the catecholamine receptors, counteracting the downregulation effects that long term stimulant consumption can cause.
These 3 supplements are vastly superior in combination with stimulant use and infinitely more safe than a benzo combination.