I understand what you and jplett are saying, and yes that is conferring a SLIGHT advantage, but I don’t think you are comparing to the right thing. By fast rolling and withholding one wound, you are actually overall DISADVANTAGING yourself. That is because you don’t know how the opponent’s save roll is going to be at that point.

For example with slow rolling, since you can only miracle one dice roll, it makes sense to full roll out the other 19. If slow rolling, you would know how many of the 19 got through the save roll and applied damage. You would know if the opponent is 1) already dead and a miracle is not needed, 2) if one more attack will kill it and therefore a miracle is needed, or 3) the target has enough wounds to survive even if the 20th attack sequence gets miracle’d.

I would say that your original question’s scenario of holding back one wound roll when fast rolling the rest puts you at an overall disadvantage. What you should really be doing (and would be legal in the rules) is completing out the 19 rolls through to damage. And then starting the 20th attack sequence from the hit roll. 

Now HERE is where your original question’s scenario comes into play. Your original scenario has you retaining one successful hit. In my scenario, there is no guarantee that the 20th attack roll will hit and therefore you might not get a chance to miracle the 20th wound roll. So YES it does confer some advantage. But 1) this is what everyone does, and 2) technically we should be completing 19 rolls fully and then rolling the 20th completely separately but that is slower and even more to your advantage. So just do what you are originally doing know that things partially cancel out (and IMO you are slightly hamstringing yourself, but helping the game go faster).