What are the de facto required fields in a SAM/BAM read group?
The sample tag (i.e. SM) was a mandatory tag in the initial SAM spec (see the .pages file; you need a mac to open it). When transitioned to Latex, this requirement was mysteriously dropped. Picard is conforming to the initial spec. Anyway, the sample tag is important to quite a few tools. I would encourage you to add it.