Plasterers will have no problem with protruding back boxes,
but the wall may not be so flat where they have worked
around them. It is definitely worth filling the box with
an offcut of Celotex or foam polystyrene. It makes it
much easier to clean out the box afterwards and it makes
the plastering go more smoothly.

If the box ends up protruding one or two mm from the
finished surface this does not matter because most
sockets and switches have a recess on the rear surface.

John