Just offering my experience. Hematomas are essentially a clot and are t0 thick to drain with a needle (any time you have gutted a deer you know the large clost that come out of the chest from a double lung shot are good examples of exactly what a hematoma will be like) and trying to aspirate or drain it with a needle will usually be ineffective. If the skin remains intact then you are better off letting in resorb over a few weeks or even months.
If an opening in the skin is made, and you are unable to fully evacuate the blood clot, it becomes an entry point for bacteria. Blood is a great media for bacterial growth and needle aspiration would be more likely to cause an infection than to resolve the hematoma. Allowing it to spontaneusly resorbwould be best, and it may not fully resorb, but should not be harmful.
If it is at an area that is problematic or looks infected then an incision large enough to remove the clot should be made (usually at least an inch long). Antibiotics are unnecesarry unless infected, but if infected it must be drained.
Just my 2cents.
Jeff