Can a reservist use more time off than is strictly needed to cover payroll deductions?

Yes. In recognition of the financial obligations associated with military service, a reservist may elect to use additional accrued time off, as long as the total pay received (military supplemental pay plus time off) does not exceed their regular rate of pay for the period. 

Note: If a reservist requests to use accrued time off, the department determines whether compensatory time (if applicable) must be used before annual time off.