As said, the advice these days from most decent plug manufacturers is that it is not necessary ....... however despite 40yrs in the auto industry I do still apply some thread lube.
It is generally not really advisable to use copper based products in alloy heads, too many dissimilar metals going on together can lead to electrolytic action. Steel/brass/iron/copper work fine together, add alum and it becomes a battery.
Much better to use aluminium based anti-seize if you can, I use a Loctite product but others are available.
Having said that, the effect is usually minimal if the application is essentially dry (like spark plugs) so in practice usually OK.
Just use very sparingly and if using a torque wrench reduce the torque values by at least 20%, figures are usually given for dry threads. I prefer to tighten plugs by feel, even though as an engineer I usually emphasise the importance of using correct torques. You can usually feel when new crush washers bottom out, then they are tight.
I just treated one of my bikes to new plugs. Having checked the records I realised they had been in there since 2003, that's just 20yrs and 40k miles. They have been out during that time, but only 2 or 3 times I think (PITA to get to, so only usually when checking valve clearances). I used alum paste on the threads and they came out easily.
