I think I know what you're asking, but correct me if I'm wrong: Are you asking if you need to seal the map before it's complete? If so, then no, you don't HAVE to seal the map, but it's good practice to. When the map is open to the void, you get what's called a 'leak' (pretty apt name, eh?
). The void itself doesn't get refreshed, so you get that ugly HOM effect. Not only that, but VIS, the program that splits the map into pieces so it doesn't render the whole mess at the same time (which save CPU resources), doesn't run, which is bad. Leaks can be hard to find, so it's a good idea to hunt them down when you see them, so they don't create problems later.
If it helps you design the map's geometry, you can leave the top off in Hammer to get a nice sky view, then quickly cap the 'cup' and compile, so you don't get the leak. Just always always always make sure that the final version is leak-free so that peoples' PCs don't melt when they try to run your map