Is it dictated by the customer? (I could see a customer being unwilling to pay for He for a 100 foot END)
Or by OSHA?
Or is it up to you?
There is a big difference depending upon the commercial contractor and where the work is being undertaken. For example, OSHA allows the commercial use of SCUBA; IMCA (International Marine Contractors Association) doesn't. The IMCA Code of Practice is much more restricting, however not all contractors conform to it.
So answering your question is difficult. The Diving Superintendent (and in-turn the Diving Supervisor) is responsible for assigning the gas to be used and is ultimately responsible for the safety of the diver.
As a Diving Superintendent, I've never had any Client complain about the use of Helium so its has never been restricted. Big Oil doesn't consider such costs to be an issue. Although saturation projects do "reclaim" Helium in a closed-circuit gas system to increase safety (insure supply) and lower cost (imagine the cost of Helium for 6 divers at 1300' for 14 days and an addition of 14 days of decompression!)
There are some similarities between recreational and commercial diving when it comes to a comparison between air and mixed-gas Divers. Generally speaking mixed-gas Divers are the few, the elite.
In most instances, the commercial mixed-gas diver has paid his dues by having mastered their occupation in the "Air" diving environment. This is not necessarily true in the recreational field. New recreational students get into Nitrox immediately after certification and then move on into Trimix. This is why they have never developed a need to deal with narcosis and why it is such a danger to recreational divers today.
The danger has always existed, but years ago, the majority of recreational divers didn't have a choice. This in one way, was more similar to the commercial industry. Most commercial diving companies make their livelihood by using air only. Divers are subjected to depth for extended periods of time and subsequently, they learn to adapt or they leave the profession.
By the time they make the move to mixed-gas, they have the experience necessary to move to the next step. Nothing comes easy.
Some might feel that they are cowboys, but no more than other professionals that make their living by doing something that's potentially dangerous such as rodeo riders, loggers or downhill skiers. They don't just decide one day to do it and 3 weeks later, there they are, saturated at 1000'. Sorry for the ramble...