I think it is all about who has the ipcs to spare from existing factory builds. That may be ANZAC, Russia, UK or none of them. It depends on what is happening in each game.
The 3 unit max ANZAC build in Australia can be a constraint if meeting national objectives in the east Indies, in which case a second factory is an idea and options are limited. In such circumstances I have started building bombers, as J is likely to already be on the back foot at land and sea, but might look for a second factory option in future.