you have to do a c check anyway weither it is in 6months 11 months or right away, advantage to doing it immediatly is
a) you get the plane slightly cheaper
b) you've got a clear 12 months running.
Any competition coming onto your route will no doubt at some point have to break for 2-4 weeks for thier c check giving you the lead on route image.
*edit* only down side is as i see it, is having to wait a bit longer to save them pennies for this extra cost, although if you order it without c check it will go into maintainance automatically and put you in the red.