Insulating your home and 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)