You don’t add any wind corrections (additives) with a tailwind, but you should still use a minimum of Vref +5kts (Ref 737NG FCTM, Oct 31 2008, Page 1.3). The FCTM now has a good table giving examples of Reported winds, Wind additives & Approach speeds to (hopefully) make it crystal clear.
Some people think that you can approach at Vref+0kt in a tailwind. I think this myth came about from poor wording in early FCTM’s. The earliest copy (B737) I still have is dated FEB 28/90 and reads:
“Do not apply wind correction on final approach speed (VREF) for tailwinds.”
The current FCTM is now more clear about this and reads:
“Do not apply wind corrections for tailwinds. Set command speed at VREF+5 knots (autothrottle engaged or disconnected).
S&L