I find it makes a difference, but only a select few carbon posts have much flex.
My favorite is an older Specialized two-bolt w/ "zert" insert. A bit hard to find now.
The more current Specialized with one bolt is good as well, but the pivot mechanism is a PIA.
Velonews did a test in 2012 that was useful:
The FSA one is also hard to find- many (some at least) broke.