I think preset tags would be a better idea because you'd run into people using many different ways to label the same thing, such as "Futuristic Space", and not so intelligent people using tags like "scence ficton" or "Sy Fy". I think it would be less work if there were preset tags, but there was a field where you could request new tags that would have to be approved by a mod. If they think it is a unique tag worth being added, the mod would be able to change the name to something better if the name they suggested wasn't too descriptive, and create the tag automatically applying it to the build that suggested it. Mods could also create tags without builds at any time obviously.
In addition to helping make similar tags the same, it would cut down on tags that aren't really worth being tags that newbies spam because they think the more tags they can spam the better it is, such as "road" or "brick" in a city build.