Unfortunately, there's no "one size fits all" value. There are several issues to consider.
First, from an autogen suppression (avoidance of) viewpoint, where is the edge of the bounding box (you have to envisage it) relative to any expected autogen? Bounding boxes are oriented in cardinal directions. Depending on the orientation of the GPs, a bounding box can have a lot of "wasted" space (i.e., space not occupied by GPs). By breaking your GPs up into groups, you reduce the size of the bounding boxes and, hence, the "wasted" space. However, reducing group size to where it is smaller that the largest GP is pointless since the bounding box will always be as large or larger than the largest GP it contains. If you reach that point and you're still getting autogen suppression, the only alternative is to split the GP instead.
Once the concept of multiple groups was introduced, an obvious enhancement was to limit the visibility range of polys and lines within a group (to improve FPS) - especially lines. Visibility is measured from the edge of the bounding box, so multiple groups will allow better phasing in and out of the ground poly visibility.
In my own case, I generally use a bounding box size and line visibility size of 500m, meaning my lines are in view for a maximum of 1000m. I generally leave my poly visibility at "unlimited" since polys coming in and out of view is usually apparent. But, if you have a bunch of small polys and want maximum FPS, you might try reducing poly visibility as well.
On the other hand, multiple groups results in larger file size and greater overhead, so you shouldn't do it unless its needed. Any only you can be the judge of that,
GP elevation is, by definition, the elevation of the ref. point. The smaller the group size, the closer will the GPs be to the ref point. Hence, the less likely will be "ripples" in the terrain (which I believe are responsible for GPs disappearing at certain angles.