Unless you've already fired a shot at the deer standing and you're following up with another to put it down for good, I'd not shoot at a running deer. I've done the former at a big buck that was double lung shot but still took off at full run broundside at 50 yards. The second with a scope aiming at the level of the bend in the front leg went through the heart and dropped him right there for good.

But as a first shot, I'd rather wait for the deer to stop and turn for standing broadside or slight quartering away than take achance at a runningshot that could just as well wound, you track the animalfor miles and lose.