If you want to follow the factory instructions exactly, you should not put anything on the bolts (except the front one already mentioned which needs sealant). In addition you should not re-use bolts. Here are the reasons...
Bolts provide clamping action to hold things together. This clamping is a factor of the strength (roughly based on diameter) and how much the bolt is being stretched (you can think of it like a really really strong rubber band). What stretches the bolt is the threads being turn down into something like the block, or being pulled up by something like a nut. For example on a metric 1.25 pitch bolt, each turn pulls the bolt up 1.25mm.
The problem with the above is that figuring out how much each bolt should be turned is too complicated for the general public (the military uses the complex way which is technically much better). Sooo... the general public uses torque wrenches to *guess* how much the bolt is being stetched and consequently how much clamping is being provided. And this torque reading is simply how hard the nut/bolt is to turn.
What makes the bolt hard to turn? The threaded hole is initially rough, as well as the bolt. New bolts and holes are harder to turn than ones that have been used several times. Even simple things like washers can affect this, and most nuts go on with washers to provide less turning resistance which makes the torque wrench measurement more accurate.
Back to the head bolts. After the head bolts have been used once several factors have changed. 1) The bolt threads have become slightly polished/less rough, meaning that for the same turning resistance you are actually stetching the bolt more and providing greater clamping force. 2) the prior installation may have even over-stretched the bolts resulting in bolts which are actually smaller in diameter and have different clamping characteristics.
What does this mean to you?
In reality, the above probably won't really affect you much. Many people re-use head bolts (they are expensive to replace). And the manufacturer is going to provide guidance that is as safe for them as possible, and may even further increase sales of their products.
