As the other poster has stated, it would literally depend on the drilling you have in mind.

 With shifted cg placements, low topweight can sometimes be a plus. For example, if you have a ball with less than 1oz topweight, you can basically put the cg anywhere you want without having to worry about it.

 Once you get over two ounces, you have to be careful to be able to drill it out and get it legal when shifting the cg a lot. Once you get up around 4oz topweight, you start having to seriously look at drillings which leave the cg a bit closer to the grip.

 Over 5 1/2oz topweight and you limit your possibilities quite severely, forcing you to not only keep the cg in the gripping area, but also drill the finger/thumb holes deep as well.

 Depending on what you want, topweights of all kinds can be used, IF done correctly.
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Good transactions list in my profile