I'm going to use a pair of helper springs on the rear of my car, and wondered if you guys thought it necessary to use some sort of bushing/isolator/locator between the helpers and the main springs? They're the same diameter, and a reasonably tight fit around the shock bodies.

 

Use the locator or the main spring will move and might rub up against the shock body/threading. If you don't have a locator then just make something up out tubing(2.5OD ?) just to keep everything centered.