Insulating ductwork is an inexpensive and easy project to do, so go for it.

While we are on the subject:
In the average home…
35% of heat is lost through the gaps around doors and windows.
18% directly through the doors and windows themselves!
(a door or window cannot hold in the heat as well as an insulated wall)
17% through the floor into the unheated basement or crawlspace.
13% through poorly insulated walls.
10% through the ceiling into the attic.

That leaves only 8% to keep you warm!

(Family Handyman magazine – October, 2001, pg.34)
http://www.familyhandyman.com