As the nut is fitted into a slot between the head plate and the fingerboard I do not glue it in place. String pressure is sufficient to hold the nut in place and adjustment or replacement is easier. Sitka spruce, Englemann spruce, and Western red cedar. I think they all have advantages. From a builders point of view Sitka spruce is the toughest of all, I use it for all of my top bracing and prefer to use it on the larger guitars.
Englemann spruce is somewhat softer, often lighter and carves beautifully.
It works well on back braces, as back joint reinforcement and for all guitar sizes. It is easily driven and is my favorite spruce for nylon stringed instruments. As both of the spruces vary widely in stiffness, colour, and density no absolute rules apply. Western red cedar is the softest and lightest of the tone woods I use. Because of its softness cedar topped guitars are somewhat fragile in comparison to spruce.