I agree you should know your application before you do this. In most cases this should not cause issues especially using SA. That is unless you have SQL Auth turned off then I'd suggest a service account that you use to start the SQL Server Service and/or Agent.
I do know that in older versions of sharepoint you may run into issues if sharepoint expects the database owner to be the same account that runs the sharepoint farm. In that case though you may have bigger issues since that would mean the person who left your company also owns your sharepoint web servers in the farm so good luck with that.