Registering your infant in arms ahead of time is a good idea for another reason. Only one infant in arms is allowed per row due to the number of oxygen masks. So, if you call in ahead of time, in the off chance that there is another person with an infant in your row, they're the poor souls who will (probably) be forced to move when the conflict is discovered.

Also, in the case of an international flight, it is critical that you do it ahead of time. If you don't, you'll be charged 10% of the available adult fare when you register your infant. If you do that when you check in, that'll probably be a full fare Y ticket and thus much more expensive.