IMO, it depends on if the tone of your letter leans more towards "if it's not in my DNWs, go for it!" or "nothing too far outside of my explicit likes, please!"

So, if you don't really have any squicks aside from a few things it's better to list all those things, ex: "DNW scat, underage, or non-con please, anything else is go!"

If your letter is phrased more like "I enjoy enthusiastic consent, happy endings, and as far as smut goes I tend to be pretty vanilla" then you likely don't have to say you don't want scat.