I always grease bolts unless they tell you specifically not to. Greasing allows the bolt to tighten a bit more which helps with the spring force of the steel to secure it better, of course you won't want to do that if carbon fiber is involved due to possibility of crushing the carbon fiber. Grease actually fills in the microscopic gaps between the male and female threads which will make the bolt less likely to loosen up due to vibrations from riding. Greasing will prevent metal to metal bonding so it eliminates seizing due to oxidizing (rusting); and related to that grease will prevent moisture from seeping down threads and into the frame where it could start the rusting process of a steel frame. Since all my bikes are steel I grease all my bolts, mostly due to wanting to prevent rusting inside the frame and or seizing.