I don't know about breath specifically, but the tastes/smells etc. that I think I've seen associated with him the most are mint, cinnamon, herbs, cheap soap (usually given some romantic and fanciful description, but still clearly identifiable as cheap soap), MANliness, and that fanfic favorite:

"Something uniquely Snape"

Gah. Kill me if I use that one. Please. I've seen it used about pretty much every character in every fandom.