Chad,
If you have a complex SFDC, then you do need to consider which fields to sync. If you have a more standard, out of the box setup, then just sync it all.
Key fields:
- contact information
- campaign data
- subscription/privacy fields
- customer identification - who is a customer? how do you know?
- Status fields
- Source fields
DO NOT EVER sync a field with SSN or Credit Card or passwords. Marketo is not PCI compliant and it would be far too easy to prefill a form with this detail. I had a client who did that once and we had to clear out the fields after breaking the sync.
I disagree with Sydney - subscription/privacy fields MUST be synced to Marketo to build the Subscription Management Center and ensure you adhere to privacy and communication laws globally.