Before.
As you will be fixing to the floor, (I assume, I normally need to,) it will mean the carpet will have to be cut by yourself to make sure there is a good and level seating on top of the sub flooring. If the floor is slightly out (bowed) in places this will also save you the time scribing-in as the carpet will cover up small indifferences and/or packers. A carpet fitter will make the carpet look good where it meets the wardrobe but if you have to cut the carpet you will not have enough carpet to make that neater edge plus you may have to take up more carpet (and underlay) to fit new carpet grips. If you may a mistake when cutting the carpet its down to you to fix it.
If you fix directly on top of carpet its not a real sound seating and if they wish to change the carpet in the future, to one of a contrasting colour, you may get an out line of the old colour showing at the bottom where they can not remove the old carpet. Also the same if they change to a flooring of a shorter height which can make the whole thing look really silly and needed beadings could detract from the look of the piece.