I have to agree with Shawn Coyle.
The forward speed of a helicopter on ditching should be low enough as to not have any "digging in effect"
On most wheeled types (S61, 332L and S76) I would select the gear down anyway to give more weight to the lower part of the aircraft and to maximise stability when floating as much as possible.
Most helicopters are top heavy and anything which lowers the weight in the water would be positive in my opinion and reduce the chance of a capsize during evacuation.