I believe the phrase is, "if the bolt is subject to rotation" , if I recall it was an FAA test answer almost 30 years ago.
Now most Cubs, Pacers, later T crates, and my Tripacer have a tab to lock the head of the bolt against rotation. So as an IA if it has the tab, the bolt is not rotating, then the "fiber locking" nut may be used. God I sound like a lawyer. This keeps the bolt from rotating in the weld on fuse fitting and wearing this fitting out.
Anyway, I use a minnie nut in this location, the six point all metal one, because of the ease of attaching a socket and it firmly engages the bolt threads every time.
If you are going to use the fiber locking nut on your gear throw them away after about two uses; they are cheap enough. If it bothers you use a new one each time. If it's old and rusty and the insert is black and oily throw the damn things away.
If in doubt use the castellated one with a cotter pin. Better yet consult with the A&P/IA who is signing off your aircraft maintenance or who has trained you to perform your "preventative maintenance"
There are a number places I use a fiber locking nut where a castellated one had originally been used most of them being a function of the float to wheel change world I am involved with as well as one installation where a properly torqued and pinned castellated nut will work loose before the next 100 hour, a non rotation situation, and a fiber locker will not, but this is rare.
Ok, one more thing to keep in mind: if it takes a cotter pin this does not mean the bolt is meant to be loos!!!!!!! The ball rod end, say at your throttle if it takes one, this bolt is meant to be tight. A hundred places on spam cans and Beavers with KSP4 bearings-tighten the dang bolt! I replace hundreds of dollars of fittings etc where the previous mechanic left the bolt loose thinking it needed to rotate and it hogged out the hole. The ball in the bearing rotates not the bolt.
The little bug on your cub mixture and or carb heat... Ha ha ha haaa. Don't mix them up and pay attention, it could cost you your life. It takes a cotter pin, it needs to rotate in the hole, one is thick and one is thin, and that's all I'm saying.
Rocket
ps. say, where is MCS Mike on this one?
pps. I do mostly DHCII and C100/200 float birds so YMMV.