Speaking as an A not quite B cup, there is no way I'm going without support between me and the Brustfleck! It's almost as vital as for larger ladies for a sort of similar but different reason. Being small means you can easily slip under things... and also one layer of support tends to flatten more than if you have two. The Dorothea stays are brilliant for this if made in linen. The linen stretches over the bust and eventually forms cups while the boning smooths and moulds the body. There are enough examples of smooth vertical lines seen under the lacing that suggests some slats of stiffening agent used in an inner layer. I get the exact look with my single layer of silk over my stays as the silk sits in the furrows between boning.
The irregular folds and creases in other paintings are a different kettle of fish but I also don't think they represent the chemise and nothing else. If they did there would be more poofing over the lacing because the lacing digs into the flesh and thus the fabric.
Inner support also means less stress on the lacing which creates less pull on the opening which can bring the waistline up.