I suggest you look at the on-line Help entry titled "Modifying SAR Settings" for general description. Below are my answers / comments:
- it's not "necessary" to use "Gridless" definition; but it's fast and it doesn’t do additional operation of creating the voxels (cubes) from the HFSS mesh. Note that HFSS mesh is tetrahedral based and conformal to geometry. The Average SAR plot is smoother using Gridless option, use it as a quick check of a baseline.
- Q: does the voxel size I am using 1mm voxel size for averaging with the 2mm resolution of the model is correct? A: there is no right or wrong setup. Note that the field and SAR values are reported based on mesh inside the object, not the model resolution. However, it's reasonable that the voxel size is smaller than the model resolution. In general, the smaller the voxel size, the smoother the plot will be (it'll take more time). Also, it's better if many voxels fit into the volume defined by "Mass of Tissue" setup. SAR_10g with 1mm voxel looks reasonable. Another consideration is the size of the actual solved mesh compared to the size of the voxel. You can seed mesh at the area close to the antenna - SAR plot should become smoother.
I was able to reproduce the original message: it appears when Object list is not used and averaging process adding different objects with materials not suitable for SAR reporting.
If you can, file an official ticket to discuss further details.