If you hide a SFDC field post Marketo sync, the field will still show up in Marketo, but I am pretty sure if you tried to sync any field changes back to SFDC it would fail.
If you make a SFDC field visible to Marketo when it was previously hidden, it most likely will not sync automatically. The ModStamp will need to be updated for the sync to occur.
At least, that has been my experience.