Vertigo has never been good at keeping order in their presentations.  Some series have numbered spines, some don't, some jumped into numbered spines randomly.  I wonder what the masses want?  I am assuming numbered spines because it makes things easier to jump into a series blind.
 
The following are examples.
Series without numbers:

• 1st printing of Sandman
• Hellblazer (Still never fully collected)
  
• Vinyl Underground
• 1st printing of  Preacher
• The Dreaming (Never fully collected)
  

• The Invisibles (Vol. 1-6 1st printing do not have numbers... all printing of vol.7 have a #)
• Northlanders (Vol. 1 & 2 1st printing don't have numbers, I have seen numbers on Vol. 3, yet to see one without a number.)
• Shade the Changing Man (1st vol. no numbered spine, then it was reprinted with a new cover to continue collection... okay that makes some sense to the madness).
• Books of magic (This was an abandoned recollected series... collected up to issue 50, Vol.1 first printing has no numbered spine.)
• Doom Patrol (Vol. 1 first printing has no numbered spine.  Later Grant Morrison run was fully collected with new covers.)
  

• Fables (always had numbers)
• jack of fables
• The Losers
  
• Human Target (Series was abandoned collected wise with 2 trades, TV tie-in trade renumbered)
  
• the unwritten
• Animal Man
  
• Y: The last man
  
• Sweet Tooth
• The Unknown Soldier
• Greek Street
• The Exterminators
• Testament
• House of mystery
  
• The Un-men
• Loveless
• Air
• Scalped  
• Madame Xanadu
• Young liars