When commuting time varies, do we have to pay for trips in excess of ‘normal’?

Q. Our maintenance employees drive company vehicles and keep them overnight. The employees are responsible for servicing a number of stores. Some days, they report to our warehouse to pick up parts before going to their first location. On other days, they report directly to a work site ... Do we have to pay them for any time in excess of their normal commuting time to the warehouse?