'I have heard that the furnace/AC cleaning is a good idea. I have also
heard that duct cleaning is just a scam, because the filter picks up the
dirt that would be in the duct.
Any thoughts on that?'

Seasonal furnace and a/c cleaning and maintenance is a necessity if you
want to prolong the equipment life and greatly reduce a crisis . As for
ductcleaning, 'if' you have allergies to dirt, spore, animal dander,
mold, dustmites, etc....ductcleaning is worthwhile -- especially if your
home is old. There are numerous ways which contractors clean ducts
ranging from the scam way of holding a leaf blower to each register (yep
!) ...to drilling 1" dia. holes every 5 feet in the main ducts then
using special circulating brushes in conjunction with a super powerful
vaccuum on a truck followed up by spraying the inside of the ducts with
a mold killing agent. You need to find out EXACTLY HOW they do the
ductcleaning so you dont get taken . If done correctly, it can have good
benefits.