Clifford L. Blodget's
The Sewing Machine Master Guide states the upper tension should be lowered in comparison to a SS when ZZ stitching. SS the thread stitch lock is to be in the material. He indicates it's not important on the ZZ and a better stitch results. He writes that the wrong side is not important. I have always tried to match the ZZ stitch tensions resulting with thread lock in the material. Often this is difficult.
What is the consensus of QB members?
Edit: to make this Vintage Machine topic appropriate, I should state I'm working with my 206 and 306 ZZs.