I have never cleaned vertical runs because of air-flow and gravity and the two in the bedrooms upstairs do a straight drop from the floor and with a flashlight I can see they are clean, I do however clean the horizontal in all the houses I have been in if it's dirty. I just cut a big enough hole that I can stick my head in up from the bottom and then feed a long shop-vac hose down and suck up all the dust.
In this house, I have 2 holes cut big enough for me to stick my head in and look down the run, one at either end and I can get the 12 foot run clean no problem. I then cover the holes with the access panels meant for drywall that have a door. I just seal them with aluminum foil tape when I'm done.
I can see this won't work for those that just have duct lines coming off the furnace itself or flexible duct runs. But in my case where there is a main horizontal run so it's easy for me.
I found a couple of handfuls of dust bunnies in the shop-vac afterward. I don't see it really being necessary so long as you are using a quality furnace filter of at least Merv 8.
I just checked them in September (5 years later) and there was a fine layer of dust but not enough to warrant a cleaning.
I keep my circulation fan on 24/7 to keep the floors balanced in humidity and temperature too.