On 25/07/2019 21:56, Harry Bloomfield wrote:

If a box is proud, a plasterer will tend to make the plaster deeper just
around the box to get cover, which will show in the finished job. I've
seen it many times.

But the OPs boxes are only 1 - 2 mm proud of the plasterboard. Assuming
that there's to be a 3mm skim then they'll be 2 - 1 mm below the
finished surface of the plaster and there should be no need for the
plasterer to build up extra thickness at the boxes.