In general, image stabilization helps only when camera shake is a limiting factor. At very fast shutter speeds, it usually provides little or no benefit, but it also typically won’t improve image quality further.

The main case where IS can reduce sharpness is tripod use. Many stabilization systems are traditionally meant to be turned off on a tripod, because they may try to correct movement that isn’t there and can introduce blur. Some newer lenses can detect tripod use and disable stabilization automatically, but that depends on the lens.

For very high shutter speeds, there have been anecdotal reports of some stabilization systems causing slight blur, but this is not a universal rule. Practically, if your shutter speed is already high enough to eliminate hand shake, turning IS off is reasonable since it isn’t needed.

So: handheld at fast shutter speeds, IS is usually unnecessary but not inherently harmful; on a tripod, disabling IS is generally the safer choice unless your lens specifically handles tripod detection well.