I doubt there is a single definitive answer to your question. I'm relatively new to this, but I'll share my experience and what I've read. First, different plants "like" or in other cases can handle, different light intensity. So right from the start you need to figure out how the strains you are growing responds to your lights.
Another aspect seems to be that the hotter/more intense/closer you put the lights, the more cal-mag and other nutes you'll need, and the faster nute imbalances will show up and progress. By keeping them a little further away, it seems like it slows things down more -- it slows growth slightly, and slows the progression if issues. In that regard, whether you are growing in soil or hydro impacts your ability to (or at least the speed with which you can) respond to those issues.
I'm running at 50watts each, and 12" was way too close for my plants, they started to bleach. Dandyrandy is running 100w @6-8", based on my experience so far I truly believe that would kill my plants within days. But it works for him, so that's all that matters.
Bottom line is you may need to experiment and find out what works for you. I would start nice and far with the young ones, if they are babies, 36", teens, 24", and then slowly work your way closer. I'm sure others will have other advice. Good luck to you.
Edit: and just to make sure we are comparing apples to apples -- I'm using Cree cxa/cxb-3070 COBs.
