Don’t take my word for it, though, I’ve only ever had a few proofs in my life, ever! Let’s see what everyone else says, they’ll definitely know more than I do
The fields are ultra vulnerable to oxidisation but, when you come to think about it, there are a huge number of Maundy coins and proof sets that are really beautiful in their toned states!
I still think I’d want to keep them ‘fresh,’ if you have a choice that is?