Firmware when roaming

What firmware should be used when roaming on another carrier?

The home firmware, or the host firmware?

As in, using an AT&T SIM with AT&T firmware/PRI active on xx7455, and then travel to Mexico and on Telcel network roaming. Should xx7455 be changed to run in GENERIC mode, or left using AT&T firmware/PRI?

Or using AT&T SIM on Rogers in Canada, should the firmware/PRI be changed to Rogers, or left on AT&T?


Bumping this as someone out there must know what the “rule” for this is…

Appreciate the help.