For coral, live rocks, etc that requires a slower shutter speed and doesn't move much and/or at all moves, then yes. But, you can never go wrong with a tripod period.
I shoot freehand if I am using the 18-55mm lens but I use the tripod if I am using the 55-250 and shoot from about 10' away. Some of my best shots have been with the larger lens and the tripod. You will find the tripod a useful device for other styles of shooting outside of fish photography so it is definitely a worth while investment.
Depends on the type of shots i'm taking, if its fast moving fish, tripods don't really help, but if i'm taking a full tank shot or something, i do prefer to use a tripod to make sure its all spot on...