I would not go by any general rules at all. Test the product! Thermal cycle it and shock and vib test it. Got burned on a part that had 0.010" dia bumps. Manufacturer stated undefill was not necessary on their part due to bump size. I wanted to underfill the part but was overruled by higher ups due to extra cost and labor. Soon we had field failures due to bump separation on the bare die side. Interestingly enough, we tore open some of our competitors products and found the same part and it was underfilled. We now undefill all flip chip or CSPs that are bumped and we have no failures.