Some fridges don't even have a line running along the very top of them. Just do the cornstarch and vodca trick. Stay a good distance away from the line and relax! What I was actually planning to do was use the hole saw to drill the hole through the top layer, then stop and pull off the top layer and poke the insulation to check for any coolant lines.
As far as the screws for the tower go you should be able to position the screws at somewhere around 2,4,8, and 10 oclock. That way you won't have a screw as close as possible to the back of the fridge. Either way you should have lots of room to work with.