Obviously, if you are riding up and down hills your HR will tend to rise and fall. This is pretty effective training. IN the first place it will improve your hill-climbing ability and in the second it mimics the sort of interval training that will raise your lactate threshold and VO2max.
As to finding an "ideal" HR and sticking with it, that too is an effective form of training, as long as you know what the HR should be, and why.
What training is most effective depends on what you are trying to achieve, what your fitness level is, and so on. As a general rule, your training should be a mix of long sessions at low to moderate intensity, shorter rides at higher intensity, and some interval training.
If you want to get more scientific about this, at the top of this forum is a sticky thread that tells you how to determine your anaerobic threshold. That is a crucial piece of information. Armed with it you can take advantage of the workouts described in the road bike racing sub-forum of road cycling. Good luck with it.
Last edited by chasm54; 04-21-12 at 03:13 PM.