Should the sim pin setting on the iPhone be on or off?

Trasbo:

It’s indeed a personal choice.

What it prevents is loss of your iPhone from becoming a loss of your phone number.

Until you notify your carrier and deactivate the SIM, thieves will be able to place/receive calls on your number and send/receive texts. (including SMS 2FA codes)

The risk this presents varies. For a random petty crook the risk is probably low. If you are being specifically targeted the risk could be substantial.

The most obvious downside is that you have to know the PIN cold. You only get a few chances to get it entered properly before it locks.

Another downside is that it won’t self-connect to a mobile network after it’s been power-cycled but not yet unlocked. This could complicate location-finding using FindMy.

Lastly, is the additional task to keep track not only of the PIN, but of the associated PUK as well.

Perhaps others here will chime-in w/ additional pros/cons.

It is not mandatory to lock your SIM card with a PIN code.

If you create a PIN code, you should write it down somewhere.

It's actually a personal choice for everyone.

I do not use a PIN code, turn it Off.

I do keep the SIM card holder, every SIM card holder of a carrier has the corresponding PUK code.