Depends on a few things.
A. What fans are you talking about? Case Fans, CPU fan/s, GPU fan/s, etc. or all fans?
B. What temperatures are you getting currently with your "40%" setting? If the temperatures are okay then there is no reason to increase fan speed.
C. What kind and how noisy are these fans? You might not want to ramp up the RPM to high or else your computer might become too loud.