I would recommend after the rail with at least 8+ inches of hose between the end of the rail and the regulator. If you have 2 rails to supply fuel to, I would run the feed line to a Y fitting, from Y into each rail, out of rails into the regulator, if you have a dual inlet style regulator. If its a single inlet style, run from rails into Y then to regulator. The goal is to reduce pulsing in the fuel system as much as possible. each injector adds pulsing as does the regulator when it bypasses fuel.
