Find me a case of someone buying new plants and their fish getting some parasite or other illness from the plants...
I've gotten hydra from some plants before, it was a pain to get kill off.
H2O2 is, from my understanding, pretty unstable and turns into water even on the bottle in the shelf at the store. It's also pretty ubiquitous and some of us have had that bottle behind the extra shampoo for eons. because of this, I think dosing is hard to relate. It's like wkndracer says, one man's miracle cure begins another's nuclear winter. I've used it to kill of bba, with great, great success treating the whole tank. I also nuked a tank once, I didn't quite kill the fish, but i think I killed every other living organism in it, except a couple of plants. I'm not positive, but I think it was the same dosage. I know it was the same tank.
I don't really remember how much I used, but it was less than a teaspoon on a twenty gallon aquarium. I've spritzed plenty of individual plants with a little sprayer and full strength H2O2 with no consequence. I just rinsed them off about thirty seconds to a minute later. I couldn't say if they were totally disinfected or not, but after the hydra I got a little more pro-active.
Now that I think of it, I got a plant from petco once then noticed some salty looking fish. That may have been nothing though, because I never really took any action aside from worrying through a weekend.