Great question Betty!
My approach to this is going to be based on the evidence.
I have not found slave owners in my research as of yet. However, in my tree, my gr-gr-gr-grandfather was born in Virginia in 1806 - so I suspect that I may one day find evidence of ancestors in who were slave owners.
And, in the case of my brother-in-law's family, I have traced several of his relatives back to the 1870 Census - the first record of them being free from slavery. Thus, it is definitely possible that I could find them in earlier records when they were enslaved.
While these are still possibilties, my expectation is that I will find documents stating slave ownership (for example - Chancery Court cases in Virginia often contain lists of slaves that are connected to an estate). So, for example, if I find a Chancery Court document from 1823 which states that an ancestor of mine owned 10 slaves, I would create profiles for each of the 10 slaves and then add a .jpg picture of the document as a photo to my ancestor's profile. In the Wikitree picture of this document, I would link to the profiles of the 10 slaves. And, as Eugene mentioned, I could also list the slave owner/slave relationship in the respective profile biographies.
The closest example I have to this format in my research is this interesting marriage record that I found for my gr-gr-gr-aunt, which lists the Pastor who officiated the marriaage ceremony. The pastor had an interesting background, so I created a profile for him (even though he is not related to my family):
http://www.wikitree.com/wiki/Bell-5669
I then created a picture with a .jpg image of the marriage record and linked this to the profiles of my gr-gr-gr-aunt, her husband and the Reverend A.K. Bell - with a comment explaining this connection.