Although they will most certainly accommodate your request for early seating, late offers some nice advantages. Since we tend to book our cruises only a few months before sail date, we've always been shut out of early seating, yet we've learned to really like late seating b/c:
-we get 1/2 hr extra of "getting ready" time before 6:30 show (early seating is at 6)
-observed less crowded shows at 630
-rotation restaurants are not only less crowded for late, but we've experienced a less hurried, more relaxed dine...I guess b/c staff is not pressured to prepare for the next seating
-The hour between end of 630 show and 830 seating allows us some time to shop, take pictures, and stroll around the ship