Depends on what I am trying to do, although typically I will adjust my post so that I am using leg as I go down in the post. An example of when I would want to switch diagonals to use the leg correctly (or use the leg as I was going up in the post) would be leg yeilding towards the wall from the quarterline.
Are you assuming that one would be using both legs together or individually, and what would be the purpose of this leg aide? The leg can be used at different moments within the stride depending on the change you need to make. Your choice should not be influenced merely by your position, but by the effect you need to have on the horse. So - there really is no correct answer.
It really doesn't matter. If you sit when one rear leg is on the ground the other is off the ground. If you only give the leg aide when you sit you are only encouraging one rear leg to push more, and the other to get more air time. That's fine if it's what you need, but it does not follow that the other rear leg will do the same.
fourmares...... that's the exact problem that I'm having. I was taught to use leg when you sit...... now trying to change that is nearly imposible! I can't seem to co-ordinate myself to use leg when rising!
Use leg whenever you need to use leg. Sometimes you need to influence one hind leg, and sometimes you need to influence the other. If you can't use leg on either the down or the up- you will either need to sit the trot sometimes, or switch to the inside diagonal sometimes to get the job done.