This looks extremely pedantic, but all but one of the other dragon quest tags involve Roman numerals rather than numbers, so I think we should stick to the pattern. I was actually in the process of editing the tag into this question but I got ninja'd.
Since tags are meant reflect actual usage, and the title of the game is Dragon Quest VII (less the subtitle) in fact and not just styling, yes, the tag should be dragon-quest-vii, with a synonym from dragon-quest-7 pointing to it for convenience.
(It's probably “7” now just because whoever used the tag first wrote it that way. That doesn't establish any precedent we have to honour, it just indicates that a synonym with that name would be practical.)