Should I place my RO filter on straight city water or post-softener water?
Both sources are going to carry mineral deposits which may cause scaling. The pre-filters will remove anything that will clog the membrane, reducing maintenance intervals. 

Bottom line is if you want to extend the life of your membranes as much as possible, don't add a process to the supply water that's going to increase mineral deposits on the pre-filtered side and slowly obstruct flow over time.