There are some circumstances where I might change the defaults, but they are rare. If there is a very slow-to-render portion of the image and it's a small area compared to the rest of the image, sometimes reducing the bucket size can help. In extreme cases I have gone as low as 50x50. But it is quite rare to need to do this. On most scenes this would make the render slower.
Bear in mind that the number you enter is not the actual bucket size. When "allow auto reduction" is enabled it's just a maximum. It's forcing Terragen to always make sure the buckets are never larger than that number. The default is 256x256, but the renderer usually produces smaller buckets than that, especially with high-thread renders. Therefore if you change it to 128x128 it might operate the same as 256x256, but it depends on number of threads, resolution and AA.
Matt