I wouldn't say you are "supposed to take the ball on the rise" I would say that is an option. You can hit it at anytime during it's arc. Hitting it on the rise is a bit more difficult. It takes good timing, and it tends to be a bit riskier. You will get some free power from hitting on the rise, but you loose potential spin in the bargin.
Hitting at it's peak is ideal if it's peak is around waist high, because that is the point at which the ball is the most still (if it is ever still). Hitting it on the way down is acceptable as well. You can take a bigger swing, and hit with more spin, but in doing so you have to give up some ground.
So there is no right or wrong point at which to hit the ball. It really depends on your style of play, and what you are comfortable with. There are benefits and problems inherent with each, but you have to be willing to do what is neccessary based on the situation, and court speed.
It really is the biggest difference between Federer's approach (hitting it as early as possible, and on the rise), and Nadal (sitting back and taking massive cuts at it while it's on it's way down).