Using filters to save the sky is a really bad idea because the filter will lower the sky light by 1,2 or more stops but guess what? The shadows are lowered those number of stops too so what is the use of that? No use at all.
What you can do is use a GRADUATED ND filter however...now that makes sense...but to use a non graduating filter is silly at best because it buys you nothing.
And contrary to this belief that a grad filter is only good if the horizon is straight, this is wrong, and millions have proven this to be wrong information. We're talking landscapes here, right? There are few landscapes where a Grad ND will not work.