Hello Tim,
your concerns about the possibility of negative effects on auto-RP are present in the command reference notes:
http://www.cisco.com/en/US/docs/ios/ipmulti/command/reference/imc_04.html#wp1013545
A command that can help you to keep well-known auto RP multicast addresses treated as dense mode in a sparse mode network is:
ip pim autorp listener
To cause IP multicast traffic for the two Auto-RP groups 224.0.1.39 and 224.0.1.40 to be Protocol Independent Multicast (PIM) dense mode flooded across interfaces operating in PIM sparse mode, use the ip pim autorp listener command in global configuration mode. To disable this feature, use the no form of this command.
So an alternative could be to move to sparse mode +
ip pim autorp listener
I'm not sure if you need to add also
no ip pim dm-fallback
This should be tested
another alternative:
moving from auto-RP to boostrap protocol that uses hop by hop messages over 224.0.0.13 and so is not affected by
no ip pim dm-fallback
BSR provides other advantages over auto-rp like redundancy and election of the BSR router while auto-rp can only have a manually configured mapping agent
Hope to help
Giuseppe