Should I grease a carbon fiber seat post?
As the person who made that claim, the reason is that allegedly some greases can attack the epoxies found in some carbon fiber applications, causing a breakdown of the CF structure, and causing expansion which will jam the post in place.

The epoxy will otherwise not corrode, so it's not necessary for that purpose. The manufacturers also recommend you do not do it. 

I have a carbon fiber seatpost in a steel frame that I greased a long time ago before I learned about this and it isn't stuck, but I might just be lucky.

And of course, if you use a grease specifically designed for carbon fiber applications it negates all these claims, for they refer to the run of the mill stuff.