With shooting a foreground and the sky, you've got a couple of choices:
A) You can use multiple shots and then blend them together in post processing. A minimum would be two exposures - one to properly expose the foreground and one for the sky. That being said, if you're going to blend things, you may as well go big and start at a proper exposure for the sky, go under 2 stops...and then shoot every stop from there to 2 over the foreground, which will give you a lot of data to play with in post.
B) The full in-camera solution is using a grad ND filter to equalize the exposure. The hard part here is that skies and foregrounds will have different ratios based on time of day, weather, etc. You would need to carry multiple grad ND filters of various strengths to cover your bases. Even doing this, you'll likely find yourself running into the situation where the sky is 4 or 5 stops brighter than the foreground and you've only got a 3 stop grad ND. In this case, even using the ND filter, your sky will be a few stops overexposed. Hopefully, shooting RAW and being careful not to blow the highlights will allow you to perfect things in post.
C) Just shoot a single image and fix things in post. You need to shoot RAW and be very careful not to blow the highlights. The foreground will be very dark and you'll have to lighten it in post. This process can add a lot of noise to the image. Ideally, you don't use this method unless the sky and foreground are decently close together and you know how far you can push your camera into underexpose & save territory. If you know that the noise isn't that bad when bringing an image that's -3 stops up to normal, then this gives you some wiggle room in these types of shots.
Most digital photographers will go with option A because it is simply easier - less gear is required and the post pro isn't that hard. That being said, I've used ND's to get a good exposure of a scene into the ~5-6 minute territory. Shooting multiple exposures like this would take a long time, and if my scene is golden hour, then I'd end up losing out. So, if this is the case, then it makes sense for you to nail the exposure in one go using the grad ND.
