For 2009 and earlier, the best answer is both, Use pipe for coordination areas such as corridors or complex areas and use 2D for plumbing only areas such as wet walls. The 2010 release we have added the ability to slope 3D piping using an Angle of Deflection on female fittings like the real fittings. Overall, the choice is yours and you need to weigh the benefits to the cost.
Thanks
Gregg
AutoCAD MEP QA
Gregg Stanley
QA Analyst
AEC
Autodesk, Inc.