The company I work for offers duct cleaning, most of the time when you do a house you get a fair amount of stuff out, mostly dust, cat/dog hair and construction debris from the original build. Unless you do major construction or have a ton of cats and dogs I can't see any reason to do it more then every 5-10 years. Low wall mounted or floor returns will collect lots more crap than high wall returns.
Truck mounted vac is not mandatory, we have a portable unit that will do just as good of job as long as you don't try to do the supply and returns at the same time(some of the truck mount vacs can do both at the same time).
Beware of the guys with a very low price, most of the time the super deal only covers one supply or return, they then charge enough for the add ons to get their price way up there.
Safety King in Pontiac has a pretty good rep, we recommend them for jobs that are bigger than we want to deal with or out of our area or really nasty.