If you aren't using any mods (such as spigot or bukkit), for 1.12 you shouldn't have to reset your server, since it really isn't adding anything that naturally generates. Ideally, the typical reasons you restart a map after an update, is so that all the chunks contain the new items, if a new item that naturally generates in the world (like diorite , for example), and if you were on a map started before that update came out, then you'd have to go to previously unexplored areas of the map, to find it, because minecraft doesn't pre-generate the whole map, but actually generates the map, as players get to those areas, for the first time
Just to note - Spigot and Bukkit are not mods per se, but more like modloaders, like Forge; you can have Forge and play it with no mods (there are some reasons to do this, such as bugfixes which are not in vanilla; for example, 1.6 had issues with zombies causing lag which was fixed by Forge but not in vanilla until later versions. Spigot (at least) also includes optimizations). You do not need to reset a modded map either unless there is no way to update the mods (some will change IDs for example but usually you can configure them). Just don't update until the mods have been released (opening a modded world in vanilla is much like downgrading, and potentially worse), but plugins which do not alter the world are usually safe to remove until they update.