UPDATE: Sonic Craft has recently told me that they always loop the hookup wire through the little hole in the tip of the Cardas binding post before soldering. So, I apologize to those whom I disparaged.
However, Sonic Craft still commands that a 250 watt+ soldering iron be used. I can verify that the post heats up so quickly with that wattage iron that within a matter of perhaps 30 seconds the solder literally flows even more quickly than water. Just use enough to hold the connection. Do not fill up the tube with solder as I think it deadens the sound and I know for certain that doing so wastes a ton of solder. Additionally, I would say after having filled up 18 Cardas binding posts with solder (7.2 system with passive subwoofers) while using a 60 watt iron, installing them all to the speakers, then uninstalling them, heating the posts, dumping the solder, and then re-soldering using a 250 watt iron after laying down some extra flux and just enough solder to solidly hold the connection, that the improvement in sound quality is amazing.