Auto populate custom field based on assignee

Hello, 

We use a custom field called "Class" that is then integrated in to Quickbooks for billing reasons.  This class can be any number of things like:  Design, Development, UX, Design:X, Development:X, etc. 

These classes are based on a persons job role or what they are doing in the task. We want to be able to know the type of user that worked on specific tasks once things move in to invoicing, because we use a lot of contractors.  

This can be done manually, but will throw errors if nothing was entered, which is extremely time consuming to fix after the fact.  

Any thoughts here?  Another idea would be in the time tracker to auto populate the category based on a users job role.  However this could get tricky if our UX designer is doing standard "design" work.