I believe there are really two matters involved here - historical accuracy and present aesthetics.
On the whole, it would probably be historically accurate for a crucifixion victim to be naked, but that may have less to do with humiliating the malefactor than we might presently suppose. The Romans had not yet absorbed the Judeo-Christian terror of being nude in front of others; indeed, just like the Greeks - whom the Romans copied in so many areas - they would be naked to exercise or to engage in athletic competition. The only item of clothing they might wear would be a sort of thong or string tied around the waist and to the shaft of the penis to stop it flopping about.
It might even have been more humiliating to force a crucifixion victim to wear a loin cloth or some form of underpants, crudely tied in place, as that would imply that the victim was no more able to control their bodily functions than would be an infant or a wild beast. As there would have been no "loo breaks" for the victim, they would have to wet and soil their garments, increasing their sense of degradation and certainly adding to the stench of their execution.
As for present aesthetics, I believe that the model should be quite naked but, where their body has a visible (and obvious) tan line, a cloth, sarong or loin-cloth of some sort could usefully be employed to disguise this; to maximise the erotic effect, the garment should be as insubstantial and as insecurely applied as possible.
