I think Katherine Thayer says she doesn't do anything but preshrink to her papers. She's found sizing not necessary.
Sorry, I missed this thread earlier.
That information above was taken from an earlier version of my website, written when I was routinely printing tricolor gum on unsized Arches Aquarelle with never a stain. (Example: the apricot print on my main page:
http://www.pacifier.com/~kthayer/
Then Arches changed and didn't print well unsized, and I experimented with different papers and sizes for a couple years before settling on my current favorite, Arches bright white sized with gelatin hardened with glyoxal. A couple of years ago, I started updating my website, which project keeps getting put on the back burner, but I did upload an updated page reflecting my current printing method several months ago.
http://www.pacifier.com/~kthayer/html/method.html
At the same time I also uploaded a new page on sizing, that reflects my thoughts and observations on sizing:
http://www.pacifier.com/~kthayer/html/size.html
The problem with setting out answers to questions about specific papers is that papers are always changing, so you never know how long a statement about paper is going to be operational. And then there's just the Gum Anomaly Principle that what works well for one person might not work at all for someone else, and vice versa. Sorry I can't be more definitive, but like many things in gum, the answer is... it depends.
Katharine