"... in general the seat post/seat tube is a spot where you want friction and grease lowers that. Best to run dry. It is good to grease on the fastener in the collar that holds the post in the frame and to grease where the collar slides over the frame. Squeaking usually means you need grease on bolt threads somewhere. Don't grease the rails or clamps or under bolt heads, just grease the threads and tighten to specified torque. Double check the inside of the frame for any sharp edges or burrs."
While I can't say that greasing the seat post in the past on other bikes (ie gluggy bearing grease not slippery lube type stuff) has caused a slippage problem for me and has cured creaking & squeaking issues, it seems Thomson are concerned that it can lead to needing to increase clamp pressure and that can crush the seatpost enough to cause it to bend or snap. I've definitely increased clamp pressure recently but I've been running it greased for quite some time with no problems.
I'm starting to suspect that my recent slipping problem may have been the result of a slightly bent seatpost and my tightening the clamp pressure was the nail in the coffin. In future I'll treat seat post creaking with a thorough cleaning and only lube the bolts and the clamp to frame surface. Any slippage I'll regard as an early warning to replace the seatpost.
Update: It appears that the manufacturer specified the incorrect seat post diameter leading to widespread seatpost slipping problems with this bike (GT Peace 9r). The overtightening of the clamp necessary to stop the slipping would appear to be the cause of mine breaking. Check out this thread in the MTBR forums.