I'd fit them after plastering because those spring clips don't work very well after you have smacked them to get rid of the massive amounts of spare plaster that your man leaves in the box. Best leave them till after. They can be cut in neat enough then anyway.
Not sure I would want to fit them after the tiler to be honest! Although, can be done.
Can be fitted before or after I would imagine. Personal preference?
I always fit them prior - when I have to (always prefer metal and noggin).
The small front lip of the box is designed to fit flush to the plaster. Agree, plasterers can go OTT, although on a standard plasterboard wall, it's only a light skim.
Fit prior and fill with newspaper to stop too much plaster entering the box, you can also put masking tap around the front lip, that way when it peels off it removes the excess plaster. They are desinged so that when the wall is skimmed you get a flush finish, otherwise the box stands proud a few mm.
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