By definition, The flow export source interface is the sending interface for Netflow packets going to the collector.
I'm thinking, what would be its effect if I didn't configure ip flow-export source on my remote router?
Will I still get the proper flow that I want from those remote routers?
This should be configured with the interface that has the IP Address that NPM is managing the device with. When the flow is received the NTA collector will compare the IP Address in the flow packet header with the Nodes table to see if the IP Address exist in the database. If there is not a match the flow will be discarded.
If this command is not configured the device will elect an interface and it may not be the same as the one NPM is managing the device by.