I'm kind of learning my way into this school of thought, myself. Plus, who wants one more step at cleaning/feed and watering time? Once my current bottle of "raw, organic, with 'Mother'" ACV is gone I won't be purchasing more.
I do it with their daily water but I'm pretty convinced it's all habit at this stage. Don't bother at all unless you get it "raw" (says 'with mother' on the label).
The only reason I'm still doing it isthey seem to prefer it to the plain water. Supposedly it prevents worms, etc. Which I've had...