Big-ip default route - rule of thumb?

there is no best practice because no best practice for that. this completely depend on your business needs which interface to use for default route. However you need keep in mind that big-ip uses two types of routing tables - tmm for service traffic and linux for mgmt and daemons running in linux environment. Some tmm routes being injected into linux routing table by default and their metric is higher then linux native routes.

Manual: BIG-IP TMOS: Routing Administration