It's more complex than that as you want to avoid local hot spots etc. Also, you can change the layout so a void on one chip is used as part of a second chip. https://en.m.wikipedia.org/wiki/Tessellation. They can use lasor cutters which can trace out complex shapes.

It's much easer to cut rectangular chips though. Still, even if the used the same overall design and just ignore any defects over X area would boost yield.