Letterhead is a useful tool; it contains important contact information and communicates professionalism. But still, I suggest you skip it. Editors aren’t all that interested in your accomplishments outside of your literary ones and any experience relevant to your writing should be mentioned in the query or cover letter anyway. If you’re pitching a book on psychological disorders and you have a terminal degree in that area, then you might use the letterhead. In fact, that’s common practice when communicating with academic and professional journals or publishers within your field of expertise.
But for fiction or other creative writing submissions, stick to a format that doesn’t take the attention away from who you are as a writer. If your professional experience is somehow important to the creative work, you can mention this in the body of the letter. Many authors make their own letterhead including name and contact information to use as a template when writing queries and cover letters. After all, that contact information is vital for those acceptances. But what’s in the letter—or in the case of submissions, what accompanies the letter—is the true heart of a creative writing query or submission.