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.
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.