I vary my workouts more by the type of workout then by heart rate.

For example, I'm doing an intense leg workout at least once week. It's all big ring/12 work. The day after I either take off or do an easy spin. The day before I may do a fast paced ride of forty miles or so. But I won't do this leg workout after doing a long ride with a lot of climbing.

For me it's more about what I've asked my muscles to do rather than HR.

V.