Carbon in your sock should work just fine as long as it doesn't get tumbled around.
The only reason I put it in the reactor atop of my De-Nitrate is because I don't run a sock. So, for my set up, it's the optimal place to put it to ensure as much water as possible comes in contact with it.
This is a quick pic of the reactor - a Phosban 150 that I shortened by ~3 inches:
And the 24g DT it supports:
Keeping the carbon (and PhosGuard) in the mesh bag ensures they don't tumble. And makes it a lot easier to replace them.
I like the idea of the carbon in a bag in the reaction to keep it from tumbling not to mention half the time to clean the reactor out when u change everything