It's situational for me.
Rally balls, just to stay in the point, I like to hit about 5/8th speed.
Winner attempts, you can go for placement, which is hit slower, or you can go for ball speed winners, which is close to 95% maximum speed.
Defense, it's often better to hit half speed balls, floaters and a mix of spins.
But that depends how well you opponent can handle to soft stuff compared to how well he can handle the ball speeds.