This is a good feature because IDT charges extra for plates with fewer than 24 strands for a 96-well plate, or 96 strands for a 384-well plate: https://www.idtdna.com/pages/products/custom-dna-rna/dna-oligos/custom-dna-oligos

This is implemented in the Dart repo export code:

https://github.com/UC-Davis-molecular-computing/scadnano/blob/8207068eaebc8ed330261516c9ae4a7b4052ef47/lib/src/state/export_dna_format.dart#L267-L275