if the goal of the balls is to make sure you shoot perfectly straight, you can also use some pocket reducers or just put chalk on the left and right side of the pockets, and play to make sure the OB doesn't touch the chalk on the way in.
It's not a bad drill, but I think a better one is the old bert kinister one where he marks a perfect diagonal line from corner to corner on the table, sets up a long perfectly straight shot, and you must first fire 10 in with follow, trying to sink the cue ball after.. then try to fire in 10 more and draw whitey back into the corner nearest you. You do it with 0 practice strokes on your first stroke to really force you to focus. You will know if you hit off center because the CB won't go straight forward or back into a pocket. Doing this with draw is particularly tricky.