What is hitting at 100%? If you are at the baseline its one thing. If you are in mid court hitting an approach shot you cannot hit with so much power since the ball has less distance to travel before touching the court. How about a volley?
See where I am getting at?
Being aggressive with your shots can mean hit hard. It can also mean hit early. Or hit with placement. Or behind your adversary. If you hit at 100% all the time you wont be able to control the ball or your percentages. In tennis you only have to beat one opponent at a time. Meaning that you only need to be as good as your opponent. So in a given match if you feel your adversary feeds off your pace maybe you will better off playing at 60% and with that you lower your unforced errors.
So... Dont think of 100% or any type of percentage. You should always play as hard as you need to win and to control the ball.