not critical, but helpful. it is helpful for three things-it makes installation easier, if you use the old "rubber hose method" in which you take a piece of appropriately sized radiator hose from your local auto parts store and force it over the top of the porcelain insulator of the plug, then carefully thread it in by hand to ensure that it does not crossthread. The anti-sieze will make it much easier to install this way.If he did it the "old fashioned way" in which you turn it to a quarter turn past tight, then it makes no difference. If he used a torque wrench, then the anti sieze will produce the correct torque reading, and dry will not. However, this is not a big deal, as long as they're all "pretty tight".It also makes sure that corrosion does not form on the threads, causing them to sieze and become difficult or impossible to remove. I have never seen this problem develop to a level that I could not remove the plug, however, the threads will not be as smooth and clean as they originally were.Basically, do it next time, but ur pretty much OK.Hope this helps,Joeto Henrysol: for christs sake man, THE PLUGS THREAD INTO THE HEAD NOT THE BLOCK! the head can actually be replaced easily but expensively, the block, not so much.