If you have routers that are anything more than simple lans+wan, you should always use loopback IPs.
It’s best practice for networking, so get into the habit. Use the loopback IP as the official IP of the router for everything:
snmp
ssh
tunnel endpoints
pppoe local address (not as much required here, but it’s how I personally roll)
iBGP peering sessions
etc
Use the loop IP as the source for outgoing things like SNMP traps, RADIUS requests, etc.
The main reason is that if you have multiple interface IPs and you use those, then you have to account for all of them in your setups, because if the network topology changes, a different interface IP might be the source.