LOL...good question! Here's the response that I got from my coach when I asked the same thing back in the day. My question was a bit more specific, as I was having troubles with maintaining my Foundation Period target ranges due to the impact of rolling terrain in the area where I typically do my rides.
"We recommend that you spend 95% of your time below the prescribed heart rate ceiling. So if you find that your heart rate is raising above this range for prolonged periods of time and you are unable to control it, I would suggest one of two options. (1) Change out your cassette to allow for a lower gear, even go so far as to have a 27t sprocket in the rear for the winter time. (2) change up the ride routes to not include the hills...though if this is unavoidable and you have no choice but to ride the hills, then go with the first option.
If you are riding at 27 now and don't want to change anything on your bike, then I would say that as long as you aren't going too far above the ceiling, its not necessarily a negative effect on your training, as we sometimes need to open the throttle once in a while on a ride to keep us sharp. This is why we prescribed Power Starts, Stomps and Fast Pedals during the Foundation Period. While we were looking for an aerobic adaptation in efficiency from the long rides spent below the heart rate ceiling, we still needed to train other aspects of your fitness.
If you are riding below your ceiling for the majority of the ride, then you are doing it right. No worries!
Take Care, Train Hard!"
Over the long haul though, my personal experience has been more along the lines of what Ric usually points out. I found that heart rate was not always the best training indicator. There are too many variables that can affect it from one day to the next - stress level, diet, weather conditions, terrain, etc. The smartest thing that I ever did was to switch to training with power. Once I did that I began to really understand how to use cadence and gearing to get the most efficient production of power. At times it meant that I had to train at a higher heart rate than expected, which in turn meant that at times I had to adapt aspects of my training program in terms of intensity or duration and to allow for proper recovery. Once my fitness and technique improved though, I found that my heart rates began to fall in line with the anticipated targets. Using that ability to achieve power efficiently and get the most out of my conditioning I was able to add intensity and duration without leaving anything on the table. The physiological adaptations that I experienced from there have been more than I ever would have thought possible, especially if I had just been focusing on heart rate.
Hope this helps. Good luck with it!