I always figure hitting the serve while the ball was at or near the peak of the toss provided me with the greatest margin of error.
That has long been the theory, because that's when the ball is not moving. It's stopped it's upward momentum, and is not yet coming down. There are some discussions about tossing the ball higher, and seeing an increase in topspin while hitting it on the way down, and we've also seen pros who hit it on the way up, very effective servers like Roscoe Tanner and Goran Ivanisovic as an example.
But the question, as I took it, was about hitting groundies. Is it best to take the ball on the rise (riskier, but gaining some free power), at the apex, or on the downslope. I don't think there is much difference between the last two options. Maybe hitting the ball at the apex can be seen to be slightly less risky because there is no up down motion, just forward motion, but hitting in on the down slope can mean you give yourself more time. Trying to hit a modern groundstroke at it's apex could find you hitting a lot of balls at or over your head which is far from ideal. I just don't think there is any added advantage to trying to catch the ball at it's apex when hitting groundstrokes.
The bottom line, to me, is you should work on being able to hit in all situations. You may have a tendency to hit on the rise, or as it is coming down, but you should be able to do both effectively, so you can take advantage of the benefits of each.