Taxi vouchers are valid for all Brussels taxis. They are treated as cash. So, drivers will give you change if necessary. For example: if your taxi ride costs €13, you can give three €5 vouchers and the driver will give you €2.
Taking a taxi therefore enables you to benefit from a sort of personal driver who knows the city like the back of his hand, who will take you where you want when you want and who can always give you a hand, and you don't have to worry about parking.