#1
When soldering guitar electronics should all wire ends be tinned or just those being soldered to the back if a pot?
#2
Yeah tin anything that you want so the connection where ever it is will be stronger and more secure.
#3
Anything that will eventually receive solder or become part of a joint should be tinned. It helps the solder flow better, you're less likely to overheat the wire/component, and it will be far less prone to corrode or become a cold joint later on. If you're worried about damaging a potentiometer, for example, you can always clip a small heat sink just above where the joint will go and that should draw the excess heat away while allowing the terminal to be tinned.
I am a StarGeezer: some call me..."Tim."*

* - Heartfelt apologies to Monty Python for blatant plagiarism. Those responsible have been sacked.
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Epiphone G-400 "Goth"
Peavey Vypyr 75 Amp