The Clean Ownership task schedule is to "correct" issues with the Asset User Owner association and the Ownership Details dataclass. This can be left at 12 hours and you probably will not see anything that you are expecting to see (clearing of Inventory) if that is what you are looking for.
http://www.symantec.com/docs/HOWTO93898
Inventory clearing is done via the Inventory Clean Up task schedule. What you want to be careful with here is what is in the 'Include' and 'Exclude' filters. For example, in my current environment, we clean up inventory on Disposed, Returned to Lessor, Retired and In Stock. In the 'Include' and 'Exclude' these resources, we have nothing. I have seen, on many occasions, that the 'Exclude' contains the All Computers filter. This will essentially cause this task schedule to do nothing because All Computers contains all computer where IsManaged =1.
Here is more information on Inventory Clean-Up
http://www.symantec.com/docs/TECH199402