Ok, so it seems that snapping to group centers can only work when snapping to nodes is disabled (and the only option enabled is snap to center).
Since 0.91, it seems that snapping to nodes is required for many more situations, so I've been keeping them generally always "on".
And the idea that certain options only "work" when other options are disabled, did not occur to me.
Even with the options set up properly, it looks like the group centers can only snap to guides or grids. Is that correct?
Thanks again,