I was told by a fitness instructor that i was making a "big mistake" to do dumbbell shoulder press with my palms facing in towards my head instead of away from me to the front

i would like to know if this is true as i couldnt see why

i also had my arms tucked in close to my body too, kinda like with the weights directly infront of my shoulders instead of out to the side

i really cant see why this would be wrong though