As a nanny, my current employers negotiated a gross annual wage.
Some agencies advertise gross annual salaries, though alas many still do net pay per hour. How they know what the net pay will be is beyond me - as every employees personal tax situation could be different... some may be paying back student loans, previous under payments etc.
You will find that mumsnetters recommend that you always state the Gross Wage in the Contract. This way if/when the tax rates change, you as employer are not affected as much, and you are able to calculate how much it will cost you to employ the person - as you take the gross annual salary and add employers NI. Also gross wage is needed for payroll purposes. I could go on... but you get the idea I hope.