I have always avoided rocks in my vivs, but probably not for any good reason. The two things that are kind of in the back of my mind are that I see frog's skins as thin and easily damaged. Maybe that isn't the case. Rocks are less forgiving than wood. I would think that nice, smooth river stone would probably be fine, while igneous rocks might be a bit more inclined to scratch the little fellas up. I have never tried it, though, so I don't know for sure. I would have thought that those broms with the super-pokey leaf margins would also poke them, but they seem to completely ignore them. So what do I know? :-)
Also, as you know from planted aquariums, there are some rocks you don't use because of their interaction with the water chemistry (I am looking at you, petrified wood). Frogs have permeable skins so I wouldn't want them to pick up undesirable metals and other stuff just from hopping around on the rocks. Again, no backing at all for this; it's just something that cropped up in my mind.
Having said all that, if other people who actually know what they are doing think that they are safe, I think that using rocks like people do in planted aquariums could introduce a whole new design element into our vivs. I can imagine an Amano-style vivarium that would look completely different than anything I have seen in vivs thus far. You would still need to pay attention to the needs of the frogs by providing hiding places (the kind of Amano planted tank I am thinking of would be short on hiding places) and other behavioral needs of the frogs, but it could be really interesting. It's actually a whole new thing (to me, anyway) because in planted aquariums, the fish can just swim around in the negative space. Frogs, being bound by gravity, can't do that, so you would have to use negative space in a different way. I think it would be super cool if those two worlds could be merged more effectively. Just wait for the buying frenzy for Baby Tears and other low-growing foreground plants following the release of your first Amano build :-)
For those unaware of what I am talking about, check out the following Google Search (the guy has an absolute gift):