Are you sure fingerprints don't count? Flatness, to a point, shouldn't matter, bur when I think back many many years to when I used a communal darkroom, 'fingerprint' could mean anything from a barely discernable smudge to a gob of peanut butter (or Marmite if you are a UK type) that would make everything into a foggy morning. Cleanliness is next to Cartier Bresson!