Is it impossible to not have shadows - for example if I have stalks of SPS there has to be some degree of shadows along the stalk yes? I'm assuming so, and the health of the coral isn't impacted by such.
One thing to consider is that the light bounces off the glass and sand to some degree so there is some light getting to the undersides and at angles. But in reality there isn't a tank that won't have shadows in some form. Some people angle supplementary lights inward from the front and back or the sides of the tank to try and avoid this.
It is impossible to stop shadows from coral growth. To the extent it affects the coral depends a lot on the species/genotype. Many corals as they grow will have dieoff on the lower sections while some will have all white nonphotosyntheitc polyps.