That's pretty much an old wife's tale. If you have a ventilated roof system, you'd have a hard time installing too much venting.In the last few years I have heard many people say that in you have the ridge vents, then you should cover up the gable vents. If not, it can disrupt the whole flow of air in the attic. The ridge vent works much better than the gable vents. Since you are having the roof done anyways, it is not an expensive addition. I would go into the attic afterwards and close up the gable vents with plywood. Doesn't have to be 100% sealed, just enough to stop large air movement.
With ridge vent AND gable vent, the worst case scenario is that air enters the gable vent and exits the ridge vent reducing the pull from the soffit vents. But you'd still be getting air movement which should remove the moisture. If you're relying on attic venting to keep the inside of the house cool, then you don't have enough insulation.