5) Only annex cities if you need to produce troops more quickly and you have a surplus of resources to do it. If after a city has produced a unit, and you have to wait for a while to get the resources to produce another unit, chances are you don't need to annex any cities. To do so requires a lot of resources!
Okay there are some exceptions to this rule. If you are a landlocked country who has expanded and wants to produce naval units, of course you need to annex a coastal city. Or like when I was South Korea and I was always low on fuel. I would have to annex fuel cities (after building level 5 arms industries of course) in order to sustain a reasonable fuel flow.
You should also use annexe to close your production gaps:
You need more electronics, that you can produce? Annexe cities with electronic production.
Also cities with the "2x" sign you should pretty much always annexe, because they produce double amount of resources, so annexing gives there a better return.