It widely depends on the situation. In my games i tend to raze a lot, and so i usually grab embarassing numbers of slaves. More are produced when an enemy city is captured and purged from the native population.
Now, condidering the need of the moment, those slaves may be kept forever and national workers may be joined back into the cities. This usually happens when i'm on a tight budget. But once the civilization has been wiped out, those slaves become harmless, and so i can as well keep the nationals and join a big part of the foreigners into friendly cities, to have new cities reach quickly size 6 (WLTKD), or simply to optimize the score with the double increase in population.