You are automatically checked-in at T-36. So, whether you actually log-in at T-24 or T-2, you'll end up with the same boarding number.

The order goes...
1. Business Select (1-15)
2. A-List Preferred
3. A-List
4. Early Bird Boarding Purchasers
5. Everyone else who logs in and checks in.

2-4 above are automatically assigned a number. So, you don't have to set your watch anymore. Just remember to check-in at some point.