According to the Klingon Bird-of-Prey Owner's Manual, deflection (per se) is essential for interstellar flight, but deflector dishes are merely one set of technology that can be used for that purpose. Some Klingon vessels have large, visible deflectors and others store them entirely in the wings and main body of the vessel.
The most constant threat faced by a Bird-of-Prey isn’t an enemy vessel
but the damage that could be inflicted by specs of dust. Like all vessels
traveling at the kind of velocities needed to traverse interstellar space, if
the Bird-of-Prey collided with dust, cometary emissions, micrometeroids
and even gasses it would penetrate the hull causing catastrophic
damage that would kill the crew in seconds.
These particles pose a continuous hazard to navigation, beginning
with relative velocities as low as 20,000 kellicams per hour—some
40,000 kilometers per hour. The problem becomes even greater at warp
speed. The Bird-of-Prey’s on-board navigational computers will
automatically plot a course around large objects such as planets or even
asteroids, but the only effective way of dealing with small particles is to
clear them out of the ship’s path.
Various spacefaring cultures have used electromagnetics and
radiative subspace devices to build barriers ahead of their ships,
effectively driving particles away from the flight path. Some of these
deflector fields have a very long range. This is necessary at warp factors
as high as warp 9 where the ship is traveling many times the speed of
light and yet must move particles away before the ship reaches them.
Many spacefaring cultures use multi-tonne coils and dishes to create
a sweeping deflector path. The B’rel-class employs a combination of
smaller fields generated by the plasma-powered warp wing, the close-in
field emitted by the defensive shields, plus a more powerful central field
flung ahead of the ship by the central deflector—a series of energized
plates surrounding the photon torpedo launcher