Check in now. There is no reason to wait. As I said, I got them a few days after I requested them. I imagine you'd get them no later than two or three weeks before your cruise.
There are two schools of thought on when to attach them. The cruise line says to attach them before you leave home, and some others here agree. I like to wait until I get to the pre-cruise hotel. However, they sent me 8 tags and it is just me going so I assume everyone gets 8 tags. If any got tore off on your flights you'll have extras to re-attach. Unless you are bringing 8 pieces of luggage!
But you can always get tags from the porters at the pier. They can't take your luggage unless it is tagged.