Hi Ritchie, Not sure if I follow the question exactly, but you certainly have the ability to add firewall rules to allow whatever specific traffic needs to get out, followed by a deny any/any/any rule, which would drop everything else. But you also mentioned allowing access to specific IP addresses in the Meraki cloud. If you happened to be looking at the Help > Firewall Rules page, you do not need to configure the MX itself with any of those firewall rules to allow cloud connectivity.
That's usually not something you need to worry about unless you're equipment is behind a more restrictive perimeter firewall or proxy that blocks all outbound connections by default, in which case you would refer to the Help > Firewall Rules page. That page shows you a snapshot of any/all FW rule info the Meraki equipment is expecting to have for all of the Dashboard comms, live tools, etc.
Let me know if I misunderstood the question.