I still exterminate quite a lot. Almost always when I take a highly developed city. The benefits are:

- A nice shot of Cash
- The ability to de-garrison quickly (and take more cities)
- The ability re-culture the city before the squalor kicks in

I will occupy when:

- it's a small or slow growth city (barbarian, rebel, or early game usually)
- the city is near a threshhold where a new palace becomes available
- if it's my culture, I'll be more likely to occupy

A good rule of thumb is I'll exterminate when there's a big cash payout to doing it, and occupy when there isn't. If there's a big payout, it generally will means the city grew fast, therefore has a high growth rate, therefore will grow back fast.

I don't enslave because I generally don't want a lot of new population in my governed cities.

The general idea is to keep my armies out in the field attacking while having enough people in the cities to keep the building queues full. Your military is a sinkhole for your money. The biggest economic mistake you can make is having unproductive military units loitering about.