My first daughter always slept with her head to the left. We didn't think anything of it until at her 2 month visit for vaccinations that the nurse pointed out her head was flatter in that spot. She said if the head were to remain flat and we did nothing to improve it, it can mess up their facial features cuz the face will shift forward.
She referred us to head shape specialist and they did an assessment on her and they said hers wasn't severe enough to get a helmet, which cost $1600, and she would've had to wear it for 24hrs a day for a couple months, so we were relieved.
The specialist referred us to a chiropractors office that specialized in infants. They showed us how to turn her head from side to side without hurting her and gave us tips on how to prevent it from getting worse. When she is awake, always have her upright, don't use a swing too much, don't use the infant car seat with the stroller just put her in the stroller so she is upright.
My daughter was born using forceps, I'm not sure if that affected her neck, but she didn't have the muscle strength to move her head from side to side. Doing those exercises helped, as well as tummy time so that she had more movement.
If your baby is sleeping on one side always, try to move their head to the other side when they are awake and see if they are resisting, it may be a neck muscle thing too. Cuz we never thought twice about it either, just thought she liked looking to the left.
Her head corrected itself once she could lift it more. You can't tell she had anything wrong.
Hair patches are common too as baby's move their head from side to side when laying down, or If the spot they always lay on, their hair will fall out. Not all babies, but I know a lot of them get it.
I keep finding it on the mattress, or get bits of it in my mouth when I kiss him on the head 
