I'm going to have my house re-roofed with standing seam metal.  It has asphalt shingles on it right now.  Contractor says he can put the metal right over the shingles and it's as good as tearing off the shingles first.  I was thinking it would be best if the old shingles were removed first and then put the metal on.  What's the best way?  Or is he right that either way is equal, when there is only one layer of shingles on the roof?

TIA,

Steve