Many times, athletes will ask me "which foot should I jump from?"  Generally, I simply have them run and jump to see what happens.  Beyond that though, I use the following considerations to help athletes determine their takeoff foot in both the long and triple jumps:
 
Consideration #1 - The amount of "pounding" the athlete's legs are taking.

Consideration # 2 - Inquire about athlete comfort and confidence.

Consideration # 3 - Coaching observations.

Consideration # 4 -  Overall coordination, hip flexibility, and strength of the athlete.

In my book, "Long Jump & Triple Jump: From the Ground Up" I provide in-depth ways in which to assign athletes a "takeoff leg" if you are hoping for additional support.