I had my frontal third transplant done. Even though the crown is thinning I was advised to leave it alone, as the existing hair will blend in with the new grafts that would never be as dense as native hair. As mentioned above, the crown seems to be where the results are least noticeable. With diffuse thinning, the added risk of transection of native hair seems to make it not worthwhile. Unless you're slick bald in the crown, it is probably often not worth getting it done. Many of those hard-selling Turkish clinics however will always be more than happy to take your money, give you a low hairline as well as a supposedly 'full' crown that in time will unlikely look much better than before.
I'd say weigh up on how important you feel about having the crown done, have a few different viewpoints from various clinics, and consider your remaining donor hair for future prospect etc.