I think the idea of a shooter on the grassy knoll (or behind the stockade fence) is a ridiculous one, and here's why.
When a sniper wants to shoot at a moving target, the position and movement of which is predetermined as in the JFK Limo, he is going to choose a firing point that gives him the least lateral and vertical movement, ie, he wants the moving target to be as near stationary as possible from his perspective. In that regard, the ideal firing point would be one where the target is either moving directly away from him or directly towards him.
The second floor window on the corner of the TSBD would probably have been technically the ideal spot, except that the trees on the outside just after the corner of Elm Street would likely block the view directly down Elm Street. Its also too close to the ground; people are likely to spot the sniper.
Going up to the Sixth Floor fixes those problems, but introduces one of its own, the need to track the target vertically. However, tracking a rising or falling target is much, much easier that tracking a target that is moving left to right or right to left.
The Grassy Knoll, however, is low to the ground and there are a lot of people around, which will increase your chances of being spotted before you start shooting. They will also be deafened by the gunshots and are sure to spot you once you do fire your first shot. Also, you are close to the target and on the inside of a slight bend. That would make the target most difficult to track as it is almost at its maximum left to right speed. Even worse, there, there are bystanders around, and they may not be stationary... you can't predict what they will do as the Presidential Limo approaches. One of them could pass through your firing line as you track the target. You are trying to shoot JFK and you will only have a couple of opportunities to do so; you won't want to waste them hitting bystanders.
I conclude that the Grassy Knoll/Stockade Fence area is just about the worst possible place in Dealey Plaza to shoot at the Presidential Limousine and no sniper who knew what he was doing would choose it. Instead, I think a sniper would choose exactly what LHO did. It was the best spot with the greatest chance of success.