I tried to find an explicit statement whether a new IMEI needs to be written to nRF9151.
These sources state that nRF91x1 ships without IMEI and it needs to be written:
nRF Cloud IMEI ordering service
While this states that nRF9151 has a default IMEI, and does not mention whether it needs to be changed or not:
Actually, I just noticed this at the nRF Cloud:
So, it looks like I answered my own question, I think the documentation could be updated to avoid confusion.
Additionally, if we ship the devices with the default IMEI, does that mean we should re-write the same IMEI to the modem to prevent it from being changed in the future (though it's unlikely that would happen)?