Is "should've" a proper contraction?

I think it’s proper but YMMV.

You would be so much cooler if you’d spelled that I’dn’t’ve.

Who said that “amn’t” is improper? It’s just very, very unusual, at least on this side of the pond.

I’m not much of a grammar stickler, but this one just makes me want to burn things.

I see the contractions in this thread are coming very regularly now.

“Amn’t” isn’t used much because it’s hard to say; the standard contraction for “am not” is “ain’t”.

Some opinions of proper contractions.

I disagree about it being hard to say. It looks like it should be hard to say with the adjacent m and n but it is no more difficult than shouldn’t, with a vowel of convenience placed between the consonants.