Some end users define nipples as short pieces of pipe of between 2 and 6" and require nipples to have greater wall thickness than threaded pipe. I've never quite understood this other than perhaps where nipples are used to attach a root valve to primary process piping. In this later case, you would be increasing the mechanical strength of the branch connection up to it's isolation point.
Anyway, myself I have not treated nipples differently from threaded pipe.
Dealing with threading small diameter pipe that is 12" long or less in the field is a major "pain"! ...The shorter the length the more painful. It's best to pre-determine required nipple lengths so they arrive to the project with threaded ends ready to go.
