Don't do installs anymore but back in the day yes and no. Should you? Yes. It's a very cut throat market in residential. 9 times out of 10 the low bid gets the work and I promise you his bid will not include duct sealing.
Ok thanks for the responses. I personally prefer to duct seal it with paint on mastik because its the cheapest, but sometimes i use the foil tape, i would never use mastik tape its like $40 a roll. My boss insists we do it though i was just curious because i had the theory that its not a leak spot unless you undersize your ductwork. The way i see it is that the air wants to take the path of least resistance which is down the duct to the supply or return, why would it want to fight its way through a tiny crack if the duct isnt undersized.