She may be thinking of a couple of things:
a. Save the cost of a cutting fee at the venue
b. Save time by having the slices pre-cut
c. Fear of having the servings messed up if the "cutting" will be done by her MIL, a friend, her mom, etc.
d. Simple ignorance of why this is NOT done. Ask her why and listen. Then patiently explain why it is not possible. (Cake muggles have no idea of the specifics involved in a large cake presentation/cutting/serving.)
Once you explain WHY this is NOT done, she'll probably be fine. Provide a cutting chart. Ask that the person who "may" be cutting the cake be present when you deliver. Show the person who may be cutting the cake how to do this when the cake is delivered at the venue.
I'd treat this as you would any inappropriate wedding cake behavior and tell her you as the baker have a reputation and you will not release a cake that is seriously flawed because it carries YOUR name as the baker. A cake that would be pre-cut would be seriously flawed. The guests will not know why their cake is stale and you do not wish this to reflect on your business.
If her reason is "d", ignorance, then here are some tools that you can give to the bride/venue/cutter when the cake is delivered. A paper "slice" of wedding cake(for size reference), a pair of gloves (for the cutter), a copy of Indydebi's pictorial, "How to Cut a Wedding Cake", a kitchen trash baggie (for trash like cardboard cake circles, dowels, etc.), a roll of paper towels or a 99 cent store T-towel for wiping the serving knife.
"Sample Cake Serving Sizes -- made of Paper or Cardstock"
"How to cut a wedding cake"
I'm looking forward to hearing her reason for wanting the pre-cut the cake!