Starships are fast but they really shouldn't be that maneuverable. You do still have to worry about the inertial dampeners. If you're constantly changing directions, the inertial dampeners have to compensate for the acceleration. Keep in mind that starships can accelerate at rates that would flatten anyone into bloody goo without inertial dampeners.
Even if they're always using a warp bubble to change the ship's mass, it would not be easy to compensate for quickly accelerating to 0.1c, going down to 0, and then immediately accelerating to 0.1c to another direction.
But warp speed isn't the same as impulse. Warp doesn't accelerate based on Newtonian principles. It's bending space in a bubble around the ship. That's how it's able to go faster than the speed of light.
Changing directions at warp has other issues. From what we've seen, they actually have to drop out of warp in order to make large course corrections.