When I sit up after a prolonged effort or the last interval during training, my cadence immediately drops dramatically (from >100 to ~80), and I slow down enough to get my HR down to zone one levels in ~4 minutes. I maintain that low level effort for an extra five to ten minutes, putting my total cool-down at 10-15 minutes.
Should I start trying to gradually decrease my effort/HR, or is it ok to cool down so abruptly?
That's not a cool down. I've always understood a cool-down to be a recovery-level effort. Moreover, there's no requirement that you do it. If you feel okay after rides with the way you've been doing it, I wouldn't worry.
The main reason for cool-down is the pooling of blood. If you've been cranking it out, then your heart is pumping pretty good. If you just stop then your heart is still pumping hard and gravity is pulling the blood down to your extremities. The cool-down allows your muscles to continue to contract and keep the blood moving, so the main thing is when your done with your hard workout don't just stop and sit, keep moving a little.
As was stated above, if your HR decreases rapidly, then you are in good physical condition. The big thing is that you're not just stopping. If I remember correctly the big thing about just stopping is that you get the pooling of the blood and it's a little harder for the body, muscles, etc, to recover, so it sounds like you are doing fine. Even the treadmill machines only have you do a 5 minute cool-down, so it's not like you have to do it for a half an hour or something.