Why the heel height is not a detail
Every experienced fitter gives the same instruction at the first appointment: bring the shoes you plan to wear on the day. Not similar shoes. Not heels of roughly the same height. The actual shoes, or a pair with an identical heel. The instruction is repeated because it is ignored at a rate that would astonish you, and the consequences reliably appear forty-eight hours before the wedding.
The reason is mechanical. A hem is not measured from the floor up to a fixed point on the garment — it is measured from the floor up to the point where the dress sits correctly on your body in motion. Change the heel height by two centimetres and you change the arc of the skirt across the floor, the way the train behaves, and whether the front hem clears your stride or catches it. These are not small adjustments; they are often the difference between a hem that can be finalised and one that needs to be let down, re-pressed, and re-hemmed from scratch.
There is also a posture effect. A four-inch heel shifts your centre of gravity forward, tilts the pelvis, and changes where the waistline sits on your body. A dress fitted in flat shoes and then worn in heels will pull differently across the hips and seat. The hem is the visible symptom, but the fit at the waist is what drove the problem.
The sequence that works: wear the shoes for the hem appointment — the order of fittings already has the hem last for exactly this reason, so don't compress that order by arriving in the wrong footwear. Wear the shoes in, carry them in a bag if you must, but change before the fitter picks up the chalk. If the shoes haven't arrived yet, reschedule the hem. Postponing one appointment costs an hour. Unpicking and rehanging a finished hem costs two to four hours of skilled labour and usually the turnaround time to do it.
One more thing: bustles and trains measured without the shoes will hang wrong when the shoes go on. A floor-length gown that is hemmed in flats and then worn in a three-inch heel will sit too short all round, and the back will rise in a proportion that no amount of bustle pinning corrects.
The instruction is not fussy. It is load-bearing.