I wouldn't expect any "harm" from Dex VI on any trans new enough to be spec'ed for Dexron; or new enought that later-versions of that trans were spec'ed for Dexron. Example: '66 Powerglide should run on Dexron VI, because the '67 'Glide got Dexron.
I could be wrong, and it's not like I have a '66 'Glide any more. I use Dex/Merc to save a few dollars, not to gain compatibility.
Go back far enough, into the motor-oil or Type A trans fluid days, and things might be different with Dexron VI; and GM acknowledges that.