What (if any) effects will a Salesforce Release eg. Spring 18 have on Jitterbit Integration?
None. Jitterbit can continue using the API it uses right now. The great thing about Salesforce is that it supports prior versions of its API for some time (at least 3 years from the date of release), so Jitterbit can continue to use its existing API settings. Note that new features that may be available (new objects) may not be available in Jitterbit (or other ETL software) until it is updated to the latest version.