Hi AndrewK,
After any firmware update you should carry out the auto calibration routine. You may also auto calibrate at any time to ensure the accuracy of the scope readings.
In your situation with FW2.05 installed on a new DSO, if you desire the 100MHz bandwidth, I recommend downgrading the firmware to 2.02SP2 using the '2.05 to 2.02SP2' firmware file. Carry out the changes to configure the 100MHz bandwidth, then install firmware 2.04SP1 . I recommend that you do not use or calibrate the scope whilst it is running the FW2.02SP2 as it isn't totally compatible with HW58, but if you don't fiddle, you wiill be OK. Once you have installed FW2.04SP1, carry out the auto calibration routine with no inputs connected except the mains cable. You will then have a working HW58 DS1052E DSO with 100MHz bandwidth running a known stable and unmodified OEM firmware.
If at a later date you decide to upgrade the firmware version, that is still possible by just following the manufacturers upgrade instructions. You could install FW2.05SP2 any time you like and then run auto calibrate again. If a non OS critical component fails whilst you are running FW2.04SP1, you can downgrade to FW2.02SP2, change the settings back to the DS1052E standard 50MHz bandwidth and install FW2.05SP2 before returning it for repair under warranty. Even if the fault prevents auto calibration, it will not cause an issue at Rigol as that data would be overwritten once they repair and calibtrate it. The down side of the scenario is that when you get your scope back from repair, you may not be able to reapply the 100MHz modification unless the firmware header issue has been resolved by then.