It entirely depends on how it's treated in the campaign. My characters generally tend towards neutral or evil (because good is dumb), and so would have little or no objection to slavery as a general institution, and would therefore, if beneficial, keep or trade slaves. At the same time, if similarly beneficial to me, I would also free slaves which I encountered. It's not about whether the institution of slavery is good or bad, it's about whether it is beneficial to me and helps to accomplish the mission objectives. If the mission objectives and character advancement benefit from the practice of slavery, then I do so. If freeing slaves is helpful, then I do so.
In general, however, I don't engage in the practice of actually keeping slaves, not because of any silly moralist objections to the matter, but because it's simply not practical to tote around around unwilling servants in a zone where everyone must be able to defend themselves against attackers, and therefore, against anyone trying to hold them against their will. It's not about whether it's good or bad, it's about whether or not it's practical and useful.