why do you never use the first names of Guillodon de La Saudraye?
Oh, this is an easy one! It's mostly for recognition and consistency.
Most people who know of her at all will recognize her as Guillodon de La Saudraye, or even more simply La Saudraye.
It's also much easier for people to search my blog or Tumblr in general if I'm consistently using the same name every time!
The same logic applies to Sauli. I generally just call him "Sauli" rather than "Gaspare/Gaspard Sauli," because it keeps posts consistent and makes it easier to recognize that I'm referring to the same person across different discussions.
It's also not a convention that's unique to her. We do this with men constantly without really thinking about it. People usually say Robespierre, not Maximilien Robespierre, and Danton, not Georges Danton, unless there's a reason to use the full name. The surname effectively becomes the person's identifier.
Additionally, her husband isn't really a historical figure of much note, so there's basically no risk of confusion if I just say "La Saudraye." I don't think anyone is going to stop and wonder, "Wait hmmm.. is Augustin making heart eyes at the wife, or the husband?"
If I suddenly started referring to her as Jeanne-Rosalie Guillodon, I think most people would have no idea who I was talking about. She's already a fairly obscure historical figure connected to another obscure historical figure, so using the name she's most readily recognized by just makes communication easier.
There's also a practical reason: historical sources themselves often aren't consistent about given names. Depending on the document, translation, or catalog, you can end up with different forms of the same first name (such as my example with Sauli, two different spellings!!), whereas the surname or territorial designation tends to be the more stable point of reference.
Rest assured, though, Jeanne-Rosalie Guillodon gets plenty of recognition in my personal projects. ^^
Thanks for asking!