Well, just picking 3x8 or 5x5 and using it for every exercise doesn't make sense.
In fact, using the same reps for every set doesn't always make sense. What if, when you are lifting the 4th rep feels real heavy, and you were hoping for 3 working sets? Why no lay off, rest a bit, and come back to the next set to see how it goes? What if on the last rep of your second set it feels real easy? Why not go another rep or 2? There may be reasons why, not, but if you are thinking, and allowing yourself the possibility of not being rigid about the reps, you may get a lot more out of your workout.
Our greatest fear should not be of failure, but of succeeding at things in life that don't really matter.--Francis Chan