If the settlement is struggling with it's growth and/or you want it to grow faster and/or are using that settlement to recruit many units, don't build it (for now).
It's better to build other buildings that help your trade/income and
don't negatively affect growth first. Also keep in mind that settlements with a larger populace give more income compared to smaller settlements with the exact same buildings.
In most case it's probably better to build the Emporiai later, once the settlement has grown a good deal and has a sufficient growth due to health buildings.
But if you don't use that settlement for troop recruitment and/or it has a good pop-growth (and/or a young governour that will give extra pop-growth for many years due to farm-skills or the like) then it's a good option.
Also the law-bonus of the Emporiai helps against corruption, which can eat up a settlements income to a large degree. Corruption is a big issue for settlements far from your capital. So for far away settlements that have a good pop-growth and in which you don't want to recruit many troops it's a good choice.
Hmm... imo there's no "that's what you should definitively do"-tip - it's all relative, and you have to make your own choices.
Note: only Large City settlements with Gov2-buildings can build the Apoikia (the second tier of the Emporiai).
EDIT: In case you haven downloaded them yet, here you can get the
EB building trees for all factions:
http://www.europabarbarorum.com/downloads_guides.html