Why increase or decrease rudder when using elevator in turns?
Instructors that teach that to students are fools.  You should be using rudder only as required to keep the ball centered - period.  If you are applying rudder to influence the pitch attitude, to "help hold the nose up", this is very very bad, and leads to stall/spin accidents.  

Rudder is mostly being used to counteract adverse yaw from the aileron inputs, and to some degree to correct for yawing motions caused by power changes.  But in any case it is strictly used to keep the tail lined up behind the nose in the airstream.  Anything else and you are skidding or slipping in the turn. 

If you are in a 30 deg banked turn, and are descending, the total lift from the wings is insufficient; you need more elevator input, NOT top rudder, which creates a slipping turn, which will make the airplane flick over the top into a spin if you stall while doing it.  

Don't overthink it. Ailerons to keep the bank angle, elevator/power to control the pitch and altitude/speed, rudder to center the ball; just do whatever it takes.