Any company I've worked for including present, when they let the "bean counters," tell us how to do the job, profits take an immediate nose dive. Several companies went bankrupt or were caught in the nick of time.
Where I work now, in facilities, the amount of energy we waste is incredible, all because anything we want to do has to go thru them.
The 15% silver rod has been the industry standard for HVAC/R. Using a rod containing phosphorous is not recommended for steel because it can make the steel brittle and cause fracturing of the grain structure of the steel. This thread prompted me to check my brazing stock. I still have 54 1oz rods of 45% silver. I love the flow characteristics as well as the appearance of this rod. Price isn't a factor because the cost is figured into the estimate.
I prefer using 15% beacuse it had become almost universal for a/c joints. If you begin mixing different alloys you heat some until the burn and can cause real problems with the joint or the system via impurities. Also think about different fluxes if they were used. If the tech is more comfortable with 15% and had a higher success rate i say let him go for it.