Another thing: Why not just travel at Warp Ten all the time? Is it that they needed to take up a complete week to their next destination? If the next port was nearby, they would take their time at low Warp factor. If it was far away, they would hump over there at high Warp factor. Just so it took exactly a week.
Laddie, dinna ye know wha' that does to the dilithium crystals, and the skin 'o the ship? Why, Warrp Ten ... me engines, me beautiful engines [sobbing] ...
Again, in the "TGST" the reason is time synchronization, fuel, and various other minor reasons.