The smart cover is more than enough to protect the screen on the 2018 iPad.
However... if you plan on using the Pencil with the 2018 iPad, then I recommend getting a matte screen protector. Not only does it cut down on fingerprint smudges, but it offers a little resistance for the Pencil so that it feels a little more natural. "PaperLike" is a popular one but it is quite expensive. I'm having a great experience with Mr Shield iPad Air/Air 2 Anti-glare... (Air/Air 2 protectors fit the 2018 iPad) 5 protectors for $7.