Whichever way round.

Imo, it's not the act of sneezing until you've hit the choo. However, it is a sneeze even if you're only in the preliminary stages of the ah.

Here's my take on this: While sneeze -> sneezing, sneezing -> sneeze is a false implication.

A "sneeze", as commonly used in language, is applied to the entire process from "ah to "choo" (or whatever sounds you might make when you sneeze), but the act of sneezing can include only the inhale part of it.

Have you ever aborted a sneeze (there are several techniques to do this)? You can't say you've actually had a "sneeze" until you finish it.

Obvious question:

why is this even a tag

Maybe because it's the central theme of some pictures hosted here. Or, perhaps because some people have a "sneeze fetish" and would like to find images with characters sneezing.

Sneezing would work better on animations rather then still images.