For newborn photoshoots, I recommend doing them between the baby's 7th and 21st day as they are more sleepy.
Thinking about the baby's safety and comfort I always suggest natural poses.
The sitter baby photoshoot is normally taken between 7 and 9 months old.
The smash cake photoshoot is from 10 months up to 1 year old.
You can of course have your baby's photoshoot whenever you feel like. Our babies grow and change so fast we better make sure we don't leave it pass without having gorgeous pictures of those moments.