I keep my surface temperature at 120 degrees, I honestly can't tell you why a heat rock is any different, I wish I could. What I can tell you is that I have never had a burn from my basking spot, but have had them from heat rocks.
When I rescued my Nile monitor, he/ she had such a bad burn from a heat rock that I have to keep all of his/ her temps on the low side or it really irritates the scar on its belly, and tail. My vet told me to keep his/ her basking spots at no more that 100 degrees. I guess it could also vary on how sensitive your animal is, or if it has any weak spots such as scars like my Nile monitor.