Dan,
Thanks for posting on our forums. The best runners maintain very close to a 90 cadence at all intensities. It might be a bit lower cadence for Zone 1 and a bit higher for Zone 4, but it should be really close to 90. The difference, as you pointed out, is stride length for those varying intensities.
Now, that is for the typical runner, but there are some runners who run best at 85, or 95.
Regardless, there should be very little variance in your individual cadence. Whether you run at 80 or 100 rpm, stride length should be the difference in your speed.
David