[seienchin]

Mae Geri can be delivered anywhere you want it. Gedan Kakato Mae Geri (into the knee of you opponent) is a devastating and destructive technique, that will permanently disable them. You just have to be very, very accurate. Jodan kakato mae geri (aiming for the chin) is a real jaw-breaker. (no pun intended).

Mae Geri can be used with either chusoku (ball of foot) or Kakato (Heel). Generally speaking, it is difficult to penetrate effectively at chudan with the kakato (the biomechanics just aren't helping you). A good chusoku to the solar-plexus is a great technique, that can slip under the guard of somebody who is expecting something fancy.

So, to answer your question, generally speaking in basic, in my school, we used to practice chusoku, but would sometimes practice jodan kakato mae geri as well.

With all of the mae geri's the most important thing is to remember the discipline of tuck, extend, tuck and down. the tighter and faster your tuck, the harder it will be for your opponent to intercept or avoid.
Cheers