Agreed, though there are exceptions like the New York 1960 crown which - though not a proof - has "shiny" fields and commands a premium over the normal ones.
And don't get me started on 'mirror' fields! I remember bidding for and winning a complete 1887 Unc silver currency set at Warwick which more than one dealer there dismissed as cleaned. As the mirroring was in the fields but not on the raised elements of the design and legend, it obviously hadn't been cleaned. Oh well, my gain...