For PPrune Fan #1:
It doesn't matter what size helicopter we are discussing here. (Size never matters... right?)
Even small helicopters possess positive dihedral effect.
Part 27 reguires no perceptible negative dihedral stability for IFR certification.
So does Part 29.
The US Navy and Army specifications require positive effective dihedral, even with stabiity augmentation off.
The good news for those designing a helicopter is that (for all the reasons Nick has stated above) most helicopters will naturally exhibit the positive dihedral effect. However, major changes to the shape of the aircraft, such as the addition of floats, may affect this issue.