Accuracy is not the main issue - the main issue is arrow flight time. Even if you are accurate at 50 yards, a deer can move quite a bit in the time it takes the arrow to get there. Crossbows are pretty loud, and deer have a tendency to jump the string.
For me, the closer the better. If you are accurate at 50 yards and under, you know the range, and the deer is relaxed - i.e. head down, feeding, then a 40 yard shot isn't out of the question. But for me, I limit my shots to 30 yards max- preferably closer, especially if they are on edge.