It's been said before, but the rate an employer pays his/her operatives is none of my concern, I am not a party to that contract and the terms of that contract do not concern me. The operatives contractual terms are a matter for the employer and the operative, not the customer.

If someone is just 'doing their job' there is no never any reason or need to tip.

However if someone gives truly exceptional service a small tip may be appropriate, but it is strictly a voluntary gesture from the customer.