Reasonable Hourly Increments (T&M)

The Acquisitions Department awarded a time-and-material contract for project management support to be performed for a base and two option periods. When billing, the contractor invoiced for the exact time and hours that an employee worked. Is it acceptable for a contractor to bill in different increments, i.e. 5, 8, 12, etc., on a time-and-material contract? Is there language to support this and language to include in the contract that restricts the contractor from billing different increments?