I harvest all my purple stripes, porcelain and artichoke at the same time. I haven't noticed enough difference in the timing to warrant separate diggings. I have no Creole or Rocambole to speak of.
As you said, the Turbans and Asiatics come out earlier.

-Mark

Probably the best answer to the question can be found in the link below. It is not only a question of which ripens first, but in knowing how to recognize when each type is ready to harvest.

And since the site also has photos & descriptions of every garlic in SSE's collection (just click on the 'Garlic' button) it is good garlic-flavored eye candy. ;-)

Here is a link that might be useful: Heirloom Vegetable Archive - Garlic FAQ

Last year my Blossom turbans came in a month ahead of the hardnecks. I expected Silverskin to run late, but it was very much ready about the time the hardnecks were scaping. I have several varieties in my hardneck collection, from big Music to mid-size Appalachian to a small Korean strain, and they all come in at about the same time.

For big scale market farming I am sure economics and convenience are factors, as MRAK said. But for me, the BACK-YARDENER simple indication is to watch for the leaves pattern. With about 100 plants I can manage to take care of that.