I don't go down on women lol, but my partner has experienced this before. I tell him to take a break, but he usually doesn't want to. He can usually relax and since it doesn't take much to please me, he normally can lay in a fairly comfortable position, without neck movement, but it stiffens sometimes.

I've heard some complaints about a sore neck after my guy went down on me, but in better positions he said nothing about pain afterwards.