Two additional tips that just occurred to me today (as I was turning another).
First, before you part off the sphere to start turning with the cups, hold one of the cups on the sphere and make sure the sphere seats securely in the cup. If it doesnt, correct the curve now, or you will be chasing the shape as it WILL shift in the cup while you are cutting.
Al Hockenbery recommends using a slice of PVC pipe to hold up to the sphere to make sure you are properly rounding it.
This is a finished sphere, so it obviously fits well, but this illustrates the point. It doesn't have to be a perfect fit, it just can't wobble in any direction.
Second tip. When you get to the point of sanding, make a mark on one of your chucks. Then when you need to rotate the sphere during sanding, align the mark to the same place on the lathe each time you stop. Also, get in the habit of moving the sphere in the same direction each time you reposition it. This is more important as the spheres are closer in size to the cups. With the oak sphere above, I rotated it a little less than 1/4 turn each time and had to sand in three different positions to get it all.. You could always make different sized cups to better fit a size range, but I was too lazy to make more and used these cups for everything from 1.75" to 4.5" spheres.