Too error is human but to really screw things up takes a machineWhen it is something out of your control, then it's a matter of trust. So much of reefing is.
Do you trust that manufacturer "A" put the right chemicals in its salt/additive/etc. Checking is way to expensive.
Do you trust that even the standard you are using is accurate? Depends on the company, IMO. Hobby companies are always suspect until they somehow demonstrate and earn our confidence. Big science supply and food supply companies are much less suspect.
A bottle of Morton's food grade sodium chloride or Mrs Wages pickling lime or an Oakton conductivity standard or 0.1 N HCl from Fisher Scientific is highly unlikely to be substantially off, but is every ingredient in a commercial two part or salt mix actually added correctly?