woochop,
Per NFPA 13, 2007 Edition,
8.15.18.2 Return bends shall be connected to the top of branch lines in order to avoid accumulation of sediment in the drop nipples in accordance with Figure 8.15.18.2.
Isn't that section when you are taking supply from a raw water source? I have never seen return bends used simply because the system was an AF, but I guess it could make sense.
8.15.18.1 Unless the requirements of 8.15.18.3 or 8.15.18.4 are met, return bends shall be used where pendant sprinklers are supplied from a raw water source, a mill pond, or open top reservoirs.
If you're not taking supply from any of those sources, return bends are not required by NFPA 13.
You only need return bends on dry / preaction systems if you are trying to avoid the expensive dry pendent sprinklers and both the head and the piping are in heated areas.