If you are responsible for leaks in peoples homes, helium may be usefull for those tough to find leaks, but I doubt the return on the investment for these small leaks will be worth it to a water purveyor.  A plumber needs to find these leaks as they will cause problems with the project they are working on, not because they want to save a few gallons of water.
If you are proposing testing your distribution system, I suggest checking the price of helium, this will give you a better picture of the investment needed for 4, 6, 8-inch butterfly valve and larger mains.  A typical good distribution system runs with anywhere between 5 to 15 percent unaccounted for water.  Finding all the leaks (100%) would be nice, but once found, you will go broke fixing them for the relativley small amount of water saved.
I would recommend getting a good water engineer to assist you in understanding the value of keeping your water from leaking vrs. reducing your unaccounted for water to such an extent that you have no return on investment.  Water is scarce everywhere in the world, but quality water suppliers are even more scarce.

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