spaz is right on. but just to clarify, droop typically refers to the additional downtravel the tire has from static ride height, not the overall suspension travel. sometimes if your car does not have droop limiting screws in the arm, you must limit the down travel of the arm via internal spacers under the shock piston, so it is often necessary to have your shocks mounted.
the best way i've seen it explained is the amount of uptravel your chassis has from the ride height position. hold the front or rear tires securely to the ground and pull up on the related shock tower. if you have 5mm droop, that means that end of the chassis will be able to rise 5mm over ride height and still have both tires contacting the ground. that's how i set mine. if i want to run 5mm ride height, and i want to run say...3mm droop, i will set the ride ride height first, then set the chassis 8mm off of the ground and screw in my droop screws untill each tire is barely in contact with the ground. so then when i set the full weight of the car on the ground, it should "droop" 3mm untill the springs hold the car up at 5.
was that any further help?
oh, as long as you are getting the ride height you desire using the clips, there's no reason to change to the threaded bodies. i like threads to quickly compensate for tweak that a chassis my sustain. i see a lot of guys that run the tc3 using the threaded bodies and measuring the distance from the bottom of the cap to the top of the collar to make sure everything is close. if your chassis doesn't get tweaked, or the tweak doesn't change, it's often easier to just use clips than to have to measure that gap every time you change your height...or tires.