The harsher the angle difference is between two smoothed polygons,
the more severe the lighting gradient will be on your low poly, as you illustrated in your image.
These gradients will be baked into your normal map to compensate for the low poly's vertex normals
compared to your high poly's surface normals and can cause visual artifacts after texture compression
and mip mapping happen. So if your low poly is showing gradients, consider splitting the smoothing
and UVs there.
Personally, after 45 degrees I start to consider splitting smoothing groups,
although polygon size also plays a part in my decision.
Smaller polygons won't have much of a gradient in texture space and won't be as noticeable as
huge polygons with gradients covering everything. And 90+ degrees always gets a split
because the gradients are really bad at that point.
