I know a RO string is all that is required for the Telnet protocol to backup a device config.
I believe a RW string is required for the TFTP protocol to backup a device config. Correct me if I'm wrong.
Is a RW string required for the SSH protocol to backup a device config?
Is a RW string required for the RCP protocol to backup a device config?
Some type of read-write or privileged credential is ALWAYS needed to access a device's configuration. For telnet/SSH, this is enabled mode (e.g. privilege mode 15). For SNMP, this is a read-write community string or SNMPv3 username that has write access to the appropriate OIDs.
For RCP/SCP, the user must have enable access to the device. That is, when that user logs in interactively, they must go to an enable prompt immediately without needing to explicitly enable themselves.
"RO/RW string" has no meaning with telnet, SSH, TFTP, or RCP.