You can call closer to your cruise date. I called about a week before and requested a particular rotation (I wanted to be in Triton's on formal night). If you want to sit at a table with just your family (and yes, they can and very likely will seat you with another family, particularly if yours has 4 or fewer members) you can request it then as well. You definitely don't need to call right now, there's no rush on that.
Just make sure that you have the dining *time* you want requested correctly right now. If you want early dining and it's still available, you have to make sure you request that, because it does book up.