I keep them puppet until
- they reach 7+ population
- a few good +production tiles are improved [mines,lumber mills,]
- a few good +food tiles are improved to make sure few people will be wasted providing enough food .
- there is enough empire +happiness [+15 ?]
Annex -> buy a workshop if in the money, start producing the courthouse -> micro lock the people for maximum production. This is easier to do during a golden age because of the production bonus.