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
  

A year plus late but nice blog. Numbering is helpful, especially to the casual reader who doesn't flip a couple pages to find out what issues the trade is composed of.
I've goofed on some series and read them out of order.I like that Fables has kept the numbering so far but I wish they had kept up with the little symbols above the numbers like on Vol.11-13.

It angers me that Hellblazer doesn't have numbering.

As a former retailer I have to vote strongly in favor of numbered spines.

DMZ and 100 bullets have numbers. so does the losers... well the other printings before all the trades were merged into 2 huge books. depends on the printings. and if they do not, they routinely have a play on the numbers in the titles of the books, like 100 bullets. first trade: first shot last call. 4th trade: a foregone tomorrow

I don't care that Hellblazer doesn't have numbering. I read it randomly and it's kinda fun :p
BB