Well we know for sure from experimentation that some molecules don't obey the octet rule, and a whole class of elements, the transition metals, obey an 18-electron rule rather than the octet rule. But the general principle that certain electron orbital configurations are more stable and lower-energy states, and therefore atoms will readily undergo reactions to achieve those states, generally holds true. The least reactive elements on the periodic table (the noble gases) already obey the octet rule, and the most reactive elements are those that only need a little energy (being only a few electrons away from a noble gas configuration in either direction) to get to such a configuration.