At one time,I believe there was an FM requirement for wet pipe to slope for drainage. I don't see the requirement in the most current version of Data Sheet 2-0. The the requirement go away?
I believe edition 2010 is the one you are looking for. See 2.5.2.4 which did not apply to any specific system, rather all. Since 2014 the clause was change to read "Pitch of Dry-Type Sprinkler System Piping".