this is one of those points, in the USA, that has been clarified in the last 10 years,and it has been litigated recently. If your Techs fax their paper in and/or provide time reporting/work online and leave from the house to their first customer's location, while I ain't no lawyer, they are deemed by the IRS to have a home office and they are to be paid as working when they leave the home office. The relative location of the service facility they are dispatched from has no effect on how they are paid, and should not be considered in the calculation. in the United States the National Labor Relations Board (NLRB) and/or your State Department of Labor, can confirm and direct you on this.
As a "bottom line" to the question of how far is too far, I would ask how far may you expect them to travel to a customer? Do you have or hire techs and that reside outside of what may be called your company's 'geographic area of responsibility'?
Does your company have a written policy about how far is considered acceptable for travel and return in 1day, before the company will pay for overnight accomadations and meals?