Im going to say that I didn't pre warn DS at his 4yr needles. He had to have needles in the hospital after he fell off a trampoline and they thought he had done some damage. So he would know what is in store and would have not been ok with it.
He is like me and the more he thinks about something the more he stresses out and it would have ended up being a horrible experience if I had of told him.

Though with DS2 I will tell him as he likes to know whats going on. I think that each child is different so its up to how you think you should go about it!