Depends a bit on what stop you'll be shooting at -- wide-open you might be fine with most diffusion on an 18mm if you are not focused near minimum. But of course, since an 18mm is more likely to be used on a wide shot, you'd probably not want much diffusion anyway.
I'd stick to the Frost filters if you are prone to using very wide-angle lenses. Even they can come into focus though if you stop down too much, so bring some ND's with you. If you are planning on shooting for a "Citizen Kane" style deep focus, I'd leave any softening to post at that point.
Beyond frosts, such as the Schneider Black Frost, if you want more softening rather than glow/halation/mist, I might try the Hollywood Black Magics or HD Classic Softs (or Tiffen Digital Diffusion-FX) rather than any strength of regular Classic Softs. The lenslets in the 1/8 Classic Soft are not much smaller than they are in the 1/4 or 1/2, they are just more spaced apart in some ways, so they can still show up if too wide-angle or too stopped down.
Kubrick was fond of LowCon filters by the way, you can see them heavily in "Barry Lyndon" but also some bits of "Eyes Wide Shut" and even "Full Metal Jacket". A Black Frost or White Frost will give you some similar effects.