I had a DVT after a flight. There is risk in me flying, but my (various) doctors have recommended that to minimize this risk I should wear compression stockings, take aspirin, keep hydrated and stretch regularly (obviously) + pointy end or premium economy for more space and ability to change position and move about. Clexane was suggested as a potential, but not a must do - I have some but do not use it.
Having had a multitude of tests, I do not have any genetic disposition towards DVT, so I believe (hope!) that in taking the precautions suggested, along with my awareness of the seriousness of the issue, I am probably at lower risk than many (if not most) of my fellow passengers...
I'm the guy doing circuits around the A380 (when I can sneak through First..) ;-)
If you take Clexane as a preventative, then good for you, though personally I would be concerned about the risk of using it (e.g. having a fall and internal bleeding etc) vs the risk of DVT.
If you are aware of the risk of DVT and take suitable precautions, I'd suggest you should not need to take Clexane unless you know you are high risk for DVT? But I'm not a doctor...
I've never had any issues carrying needles on flights - nor have any of my friends or colleagues who need to do so (e.g. diabetics).