$$\frac {5^{-2}}{3}$$: \frac {5^{-2}}{3}
$$\frac{3}{4}^{-3}$$: \frac{3}{4}^{-3}
$$\left(\frac{3}{4}\right)^{-3}$$ \left(\frac{3}{4}\right)^{-3}
$$\frac{3}{4^{-3}}$$ : \frac{3}{4^{-3}}
$$\frac{3}{4}^{-3}$$: \frac{3}{4}^{-3}
$$\left(\frac{3}{4}\right)^{-3}$$ \left(\frac{3}{4}\right)^{-3}
$$\frac{3}{4^{-3}}$$ : \frac{3}{4^{-3}}
Livia was quick! Yes, the idea is that the {} group components and can be nested, and the examples given aptly demonstrate that.
And Moodle Docs that Colin has been working on, http://docs.moodle.org/en/Using_TeX_Notation, have been updated to address this....