If the rest of the project is non-ARC it's easier in my experience to write new additions that way as well. I've only been using ARC on newly-created projects because having to add -fobjc-arc or -fno-objc-arc to dozens of files in Xcode is very, very tedious.

You can do a search to filter files in Xcode as well as cmd-click to select multiple files, the double click and enter the flags once for all of them.