It mostly depends on how you want to handle things.

If you want to keep your subscribed mods as clean as possible, you can resub to the collection and use the option to keep only the mods used by the collection.

But as long as you update the modlist .cfg and sub to the new mods (if any) and of course load that modlist once in the modloader, then you are fine.

You don't always need to import your game. New things will only be added to unexplored zones if you don't, but nothing should break (unless specified in the changelog) .

Mods can update in-between collection updates, that is what you download when you start Steam. It is usually fine, but can sometimes lead to issues.
In the case of UWE 2.0 for example, you all had it before the collection was updated, and it sill worked "mostly" fine ;) The mods that were removed last update were either integrated in UWE 2.0 or Anrhaa's patches, or were simply no longer required.

So basically, to sum it up, when you see an update:

1) Sub to the new mods
2) Download/update the modlist .cfg (found in the "Load order" discussion)
3 Load that new .cfg in the modloader (and don't forget to save using "Save mod order" in the top left)
4) Check if you need to import in the "Changelog" discussion.