Interesting topic as my family of expert teenagers seem to think that its a must do to have the vents cleaned. On the otherhand I am not so sure this is really something thats needed (like someone mentioned above the night mare of the "we dont give a shit workers").
We also have an Aprilaire 2400 whole house filter on the central furnace/AC unit, plus the humidifier I had upgraded a few years ago which includes separate humidistat setting. I never realized how much of a difference that makes since it kicks on when humidity is needed instead of just when the heat is on.
FYI I also retrofitted my Aprilaire 2400 with a kit they sell to use a media filter with more surface area to capture crap. I used to change it every winter/summer season but I am realizing after the last time I changed it in November, its already looking somewhat dirty. We have always had 2 cats and a dog in here too so this might make a difference. this thread reminded me I need to check the returns too.
As for the dryer vent, I have easy access in the basement and I clean that end to end myself every 6 months without fail. Thats one thing that scares the crap out of me to have that go up in flames and with 5 of us we do a shit ton of laundry. Over the summer I changed the external part of the vent to one with plastic flappers on it since the old one was stuck open and i was worried we would fry some birds or chipmunk in there. I had to take the external clip on screen off because it was catching too much lint.
But without any real experience I am still on the side of duct cleaning is a bit of a scam.