I have heard in the trade as a general rule if you are using #12 wire
for a 20A circuit you should go to #10 if the run is over 100 ft.

The fan #14 or #12 thread prompted me to just do a quick check at one
of the voltage drop web pages.

http://www.csgnetwork.com/voltagedropcalc.html

If you use this web page and try using 16A which would be the max on a
20A circuit you get 5% voltage drop in just the branch circuit.