Yeah, I thought about doing this as well. But, the problem is that you can't predict which sections you'll have extra time on. For example, let's say that you always finish with time to spare on the RC sections. So, you decide to finish the bubbling during that section of the real exam, but the June 2004's exam had 28 RC questions! My point is that you can never be sure of anything on the LSAT, so bubbling as you go would a safe and reliable strategy.