Should the seon.saveSession() be called in order to get the fingerprints for a successfully configured session_id with the Javascript agent?

The seon.saveSession() callback makes sure that the data has been saved successfully. Fraud API requests initiated before the successful callback won’t be able to reference the collected device data. 

However, this is only applicable to the earlier versions of the JS agent, the most up-to-date version, v4 uses session instead of session_id, which contains the entire encrypted payload in a base64 encoded format, making the Device fingerprinting process much more seamless, without the need for a callback. We highly recommend migrating to the new version.