You have heard correctly. It will effect the barrel harmonics of the weapon which it will cause the point of impact to change between shots. But let me explain.
If you mount a mounting system, light or anything on the barrel you will have to re-zero the rifle with the equipment on the barrel. If you for example are standing or kneeling while firing the weapon with the new Zero set. If you rest any part of the barrel or the light and mount on something it will change the Zero as mentioned above. Once zeroed you must be sure that nothing has contact with the barrel when you are shooting. It will only change if the barrel has contact with something. That includes griping the items when shooting. When a rifle is fired the barrel actually whips! That is called barrel harmonics. Hope this makes sense!