When to use Thread Compound

Thread Compound includes items such as:
Lubricants:  oil, grease, anti seize,
Locking compounds: thread locker, loc tite
Thread sealants:  pipe sealant or teflon tape or other thread sealing tape.

Applying antiseize to bolt

There are four reasons one would apply a thread compound to threads:

1.  To prevent liquid or air leakage past the threads. Example, Tapered Pipe threads (NPT)
2. To prevent loosening of the joint. Example a thread locking compound.
3. To control friction (K) for better torque control and to meet the "conditions of torque" in the tightening specification you are using.
4. To prevent seizing, galling, and corrosion. Example, anti seize.

Your choice of whether to apply or not is governed by which of the above items you need to control.

There are four reasons one wouldn't use a thread compound on threads

1. The Maintenance manual says not to use it
2. In areas of high vibration where a thread lubricant or antiseize might reduce friction and allow the nut to loosen.
3. Thread compound might contaminate system fluid.
4. System temperature that is outside of the threads compounds temperature range. Could be too hot or too cold.

Further Reading:
How Antiseize changes bolt torque