You can put drain ports wherever you want. Ideally you want them at the lowest portion of the loop, and remember that air has to be above water for it to drain. On my loop I have several drain and air ports (I just use T fittings with a homemade plug), because I have 3 valves so I can change my cpu or gpu or move my case (external rad/pump/res) without having to drain the entire loop.
Fun fact: the reason you'll see a bunch of vents on a house roof is because they're the "air ports" for your sink/tub/washer drains. Without them or if they're not properly size you get crappy drainage.
Edited by dr/owned - 4/5/13 at 4:21pm