Really depends on how long you're going to leave the boat and the likely tempertatures while you're away.
We drained the fresh water system down this year including emptying the gas water heater completely. The central heating system is 50% anti-freeze, so left that, and the engine is also 50% anti-freeze [fresh every year], so I suppose that's why we didn't bother with the heater this year.
I think we left a heater on last year because we only bought it in Oct 08, and it was our first NB, so I wasn't too sure whether it was the correct thing to do, now with the help of this forum I feel it should be OK barring any unforseen problems.
I forgot to empty the fresh water tank [about 1/2 to 1/3 full] and the pumpout tank but as these are both below the water line their contents hopefully won't freeze enough to cause any damage.