Here's a decent blog post on the subject - PIN Groups in Cisco UCS Demystified | Stretch Cloud - Technology Undressed
The default mode is dynamic pinning. Meaning a vmnic on a service profile will dynamically select an uplink from the Fabric Interrconnect based on an internal algorithm.
I would NOT recommend statically pinning any vmnics unless you have a specific reason to do so. The system does a good job of balancing the traffic between the uplinks.
