This is the outfit test scene I've been showing that had been provoking a lot of artifacts.  This time I rendered it with Irradiance Sample Size at 20.  I can only see one major artifact, at the thigh, and maybe one other at the breast.  This is a lot better than what I was getting with a small sample radius as I was assuming (foolishly, again) that a small sample radius would automatically mean higher quality.  The other two Indirect Light quality sliders (on Dimension3D's interface) are about at middle values, and GI bounces is set to 8.  Seven minutes 25 seconds.  Pretty good, good enough for me.

edit: I conclude that the biggest single thing you can do to avoid blotch artifacts will be to make Irradiance Sample Size larger, at the expense of some precision of how GI lighting is applied.  I don't think you'd want to always enlarge it to 20, but try for some compromise between low and high value that will give you "pretty good" indirect lighting and avoid artifacts.

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