As a general rule, more granular departments are better. Consistency mechanisms like tax/foodstamp/discount eligibility and desired margin all get attached to departments. For long-term reporting & history, departments are also what gets recorded in transaction records. I might split the difference: if an existing department has multiple subdepts, move the department up to a super and the subdepts up to departments. If an existing department has no further subdivisions, then it can stay at the department level.