I confess I've always used the slightest amount of copper grease when popping the plugs back in, but it seems there's conflicting practices. F'rinstance NGK recommend inserting the plug dry.

What's your take?

I use a wipe of copper grease. My thinking being it reduces wear on the threads - a consideration on 40 year-old engines.

Is their plug really 'dry' - or does it come from the factory with something on it?
Copaslip every time for me. Can't see how it can damage anything.

I use anti-seize on my plugs, but use a lower torque(about 10-15%) to prevent over tightening because of the lubricated threads.

I think NGK plugs are nickel plated to prevent corrosion.  So unless you leave them in for 25 years and keep your bike outdoors near the ocean they figure they won't seize in the head.

IMO, not a good idea. I've seen copaslip burn on and make the plugs a real bastard to get out.

I've always put them in dry. I had a talk to an engineer at the Trades Guild and he said dry and as per Dai could cause problem getting them out. 
Regards Martin.

Never had a seized spark plug and never used anything on any automotive plug thread.
In many aircraft piston engines they call for a heat transfer grease/compound on plugs, the same stuff as you get on computer CPU heatsinks. Cannot remember the details of the grease now.

That's settled then. Conflicting 

I'm in the dry camp when it comes to spark plugs. Anti seize or anything else added to the threads can affect the heat transfer and grounding from the plug to the head.

I have used antiseize always when an alloy head is concerned. Over the years I have had to fix quite a few alloy heads that have had damaged plug threads some due to cross threading but also due to the plug seizing in the thread after being left too long before being changed or checked.
I suppose it could have a small negative effect on heat transfer but I don't think it would have any negative effect on conductivity.

The danger with lubricating the threads is that it makes it easier to overtighten the plug and strip the threads out of the head if you aren't careful. It's the reason I use a lower setting on my torque wrench when installing spark plugs.

As far as heat transfer, the spark plug has absolutely nothing to do with removing heat from the cylinder, the only purpose for the heat transfer is to be low enough to keep the plug tip hot enough to burn off any carbon deposits, but not so hot that the electrode melts. The most important factor in heat transfer is the ceramic insulator that surrounds the center electrode. That's the part that changes for different heat ranges. A film of copper(a very good heat conductor) a couple hundred thousandths of an inch(.00001") thick has virtually no effect on the heat transfer from the plug to the head.