When the weather gets hot or the altitude goes up, the air gets worse so you can't burn as much fuel so you need smaller jets. I usually only alter the jetting two sizes.
I only alter the timing when the car is over powering the track (retard it 1 - 1 1/2 degrees. If I'm wanting to set her on kill I advance it 1 - 2 degrees. Just make sure you're checking your plug condition after each run to avoid detonation etc.