TragicMonkey
Poisoned Waffles
Seriously to answer the OP.
Your preferred pronoun is your business, as is whether you choose to put it in an email siggy or not.
This rather reminds me of when I was attached to the US Navy back in the 1980s. I do not have a second Christian name, I have a first name and last name, and that's it, so when I had to fill in the paperwork to be allotted a room in the BEQ and a temporary account at the PX, the form had a box for my second name, and since I didn't have have one, I left it blank.
When I handed it in, some jumped up, officious little admin clerk gave me the third degree for leaving a box blank. I explained that I have no second name, just first and last. I could see this annoyed him, and quickly told him that there's no point blaming me - my parents didn't give me a second name! He just thrust the form back into my hands, and said... "write NMI in there". I asked what it meant, and he looked at me like I was a piece of something he needed to wipe off his shoe. He said, curtly "No Middle Initial".... at this point, I just laughed, and that didn't do a lot for his demeanor... I thought he was going to have a seizure!
I have since found out that this use of NMI is quite common in the USA... it's not even thing here, which is why I had never head of it.
I've never heard of using NMI that way. At my current employer, they put in "X" for employees' middle initials if they don't have one, because they use three initials as part of our internal IDs and logins. I was here a month before I found that out, I just thought a lot of my coworkers had really cool middle names like Xavier or Xena or Xanthippe.