Hi Dave!
I can't and won't speak for Andy Blance and Thorn, but as a cyclist with some experience servicing bikes, I'll take a stab at answering your questions.
I've had the unenviable task of removing seized seatposts from friends' bikes. It can be horrible, as the 'posts can cold-weld themselves to the seat tube and a variety of methods then have to be employed to remove the remains of the post without destroying the frame. For what it is worth, each of the seized posts had either been put in dry, or had not been greased for decades. The bikes used in a coastal climate had the most stubborn cold-welding to overcome.
I grease mine generously with Phil Wood Waterproof Grease and have had no problems in 35+ years. I do my quill stems, also, and remove the parts and renew the grease periodically.
For my Nomad and tandem, I grease between the seatpost and seat tube and both sides of the shim. I have not found the extra grease to cause the post to slip, so long as the gap between 'post and seat clamp/shim is within specs. A too-small 'post will slip greased or dry, and will require excessive tightening of the seat lug bolt, resulting in distortion of the clamp and ears.
As for greasing intervals, annually sounds like "safe" advice it would do no harm to heed. I've done mine every couple years and had no problems, but I don't live in a coastal climate, the bikes are stored inside except while touring (though mine also did fine with Phil grease when parked outside in rain all day while commuting), the roads are unsalted here, and I used mudguards (fenders) -- all of which likely extends the service interval for me. I also cover the saddle when it is wet, and the cover usually overlaps the seatpost/seat lug interface as well.
I've found greasing the shim does reduce friction and aids it in compressing onto the 'post as the seat clamp is tightened.
I reversed the shim in my Nomad so it blocked the forward-facing kerf in the extended portion of the seat tube below the clamp. I have since put it back for no particular reason. Generous amounts of grease have served to be an adequate barrier to water entry in normal use and with the shim reversed, I thought there might have been a bit of slippage -- not downward, but rotationally when the saddle was leaned against a tree or wall -- with the Thudbuster LT seatpost. It has some fairly aggressive rings milled into it as part of the machining process. These tend to reduce vertical movement, but do little to prevent or perhaps even aggravate rotational movement due to reduced surface area at the clamp. I'm still investigating. I do observe recommended/specified torque ratings on my bicycle fasteners and also on the seatpost clamp and I have not found the presence or absence of grease alone to be a factor in the post moving downward.
One observation on shims: It doesn' take much of a surface flaw to cause them to scratch a seatpost, greased or not. There was a tiny lip at the bottom of one edge of the kerf on my Nomad's shim that caused it to scratch the original post. I simply removed the shim, gave it a couple strokes with a half-round file in the offending area, and all has been well since. The same thing happened on my tandem stoker's sus-post, which also uses a shim. Check any new shim carefully with your finger before inserting a post for the first time.
Perhaps Dave Whittle or Andy himself can clarify further.
Best,
Dan.
