Actually RTG slots do show the true reels, at least when it goes to the slow mode spin when you need a specific symbol for the feature, it's quite possible to predict if you will hit just from watching the symbols that flash by.
That's true, but you can guarantee the starting or finishing position is determined at the point the random reel stop signals are sent down which is before the first reel stops spinning, and the sequence is adjusted accordingly. In fact if memory serves me right, quite often you will see the animation adjust itself mid "spin" to get the correct settling point.