I found the only way to know is to experiment. I tried a lot of the conventional taper techniques and they didn't seem to work for me. I think every runner is different. My Junior/senior year of college I found that I perfected what worked best for myself.
I always ran around 100 miles/wk during cross and found that I handled mileage fairly easily or at least better than most. I did find that running that amount created a fit body but an "unatural" one. By that I mean if I didn't keep up high volume to a certain extent I would detrain and return to a natural state very quickly.
With that being said, I basically just tapered for one week...actually it was more like one day. I never missed a run during the season and trained twice a day on weekdays and once on Sat/sun. During the final week before conference or some other big meet, I would just take my Tuesday morning run off. That was it. My body was so used to training regularly, by skipping one morning run, I was able to rejuvenate my glycogen stores and recover my muscles. I essentially carried this rest over to Saturday's race.
Why didn't I just rest the day before? It shut down my nervous system. I felt pretty slugish on Wednesdays. So Wednesday, Thursday, and Friday I would try to re-awaken my nervous system while maintaining my rested state. That meant that I ran just hard enough to wake my body up, but not hard enough to break it back down.
Hope that helps. I think the key is to realize what type of runner you are and how you respond to workouts, mileage, etc.