Hi Rob. Postfix is super-flexible. The measure of how complicated it will be to get working in FIPS mode will entirely depend on how you're using it. E.g., postfix out of the box in RHEL6 and RHEL7 (for local only) won't need any special considerations (neither would a basic mail server); however, you could start running into issues once you start adding encryption (see Postfix TLS Support).

I don't run any production mail servers and haven't ever a had a FIPS + postfix case here in support; however, if you stick to basic common-sense rules (use modern certs + keys; for bonus points, generate them on FIPS systems), I doubt you'll run into trouble. That said it's entirely possible some postfix config changes will be required too. Keep us posted if you need assistance.