Is it strongly recommended to get a duct cleaning for central air?

No unless your running the system with no filter.  Most units we pull out in residential the ductwork rarely is dirty there are exceptions of course.

Most duct cleaning needed is commercial mostly exhaust duct.   Like drier,bathroom exhaust ect.

for most families, duct cleaning not needed unless the filters get filthy dirty in three months or less.  waste of money.  other service -- spray down the external radiator with full force garden hose once a year to remove dirt [when unit is turned off, of course].  by now, the internal guts might benefit from an inspection for fungus growth [usually you find this out when it clogs the water discharge line].  external inspection -- cut plants back if overhanging to minimum distance of man height.  check refrigerant level -- not truly needed -- you'll know if it gets low because unit will run far too long and be ineffective in summer.  Leaves can be removed from inside the external unit [with power off] by unscrewing the top on top, moving it aside, and inserting your shopvac hose.  electrical testing?  not truly needed until it stops working.  Lots of this 'work' is, imho, an excuse for them to get to top of the list when it does stop working -- so they get first call on the repair work and first chance to sell you on a new unit [which many repair outfits will do].  -- grampa  [note: you can flush the water drain line by applying shopvac to the outside end, and then adding one cup of white vinegar to the inside end of the line to retard fungus growth.  use the detection device that turns unit off if the line backs up to add the vinegar.]  inspect your filters every three months during cooling season and you're done.  [replace only when they get dirty -- hold up to light overhead to see the dirt].  so far, you're doing great -- you've saved those 'annual service' fees for 8 or 9 years now.  :-)

It is only strongly recommended by companies that sell the service.  Everyone else will tell you that if you keep your filters in good shape then it is completely unnecessary.

Proper system with proper filtration, do not need it.