Do helicopters ever dump fuel before landing?
Yes

07-NOV-1971
Sikorsky CH-124A Sea King (S-61B)
Suffered single engine failure on take-off. Pilot attempted to return while also dumping fuel.

From what I've read, fuel-dump systems are more common on military helicopters.
Sometimes, a helicopter may not have enough power to hover in ground effect for a normal landing or take-off because of loss of an engine, high density altitude, or being overweight. The alternative procedure is to perform a running take-off or running landing, which is an advanced, but not an emergency, maneuver.
But if the copter is landing in a confined area, such as a small pad on a frigate in a stormy ocean, there's no room for a running landing, so its better to dump fuel to allow the copter to hover into a landing.