If the heart always represents the essence of the princess and therefore the core feeling that builds her (which is my main theory for now), I think Razor's open heart shows the simplicity of her nature that manifests in a fun-seeking game of blades.
She makes her heart seen out of the wish to feel, to express more than her body could. Steel bones are her implement to affect. They encircle the heart as something most important to it, and, paradoxically, the blades don't seem to hurt it, because she never saw fallacies in her actions. They look like a comfortable suit for needs of the heart. And her cartilage is as steel as ribs, whenever it's gone in Fury and Wild, what might mean the immutability of her character, like there's no place for further change, or the inaccessibility of her sensual part, like there's no way to hurt her feelings that would matter.
In No way out the second she starts questioning what she and TLQ are, the blades degrade. The meaning of her existence disappears with the means for it. If she's not the "one to hurt you", she's nothing but the thrill she felt. And so only her heart stays.