I have the same problem but never took it much thought except tightening it every now and then. This is my idea: The problem is related to the "torquing" of the seatpost but more importantly, that the two competing forces(front and back) are increased at the same time etc etc So get the seat seated and balanced first , then turn a full turn on the screw in front , followed by a full turn at the back etc etc until they have met the torque settings. You could have the correct torque front and back but that doesnt mean the seat is resting correctly on the front and back resulting in a squeek when you put some weight on it. The rear part of the rail might be touching but not really resting which causes the saddle to "hinge" from the front and this causes the squeek. See my advanced graphics: It might be touching and torqued, but not resting.
