Depends on where your home is located! If next to a large body of water or in a flood plane, abesolutly! Not only should you insulate the duct work, you should seal the duct work as well! Coastal Insulation offers two ways to do this: One is seal with mastic and wrap the ducts with R-8 or R-11 duct insulation.
Our prefered method is to insulate and seal in one shot! We can do this by insulating with closed cell spray foam. The spray foam not only seals the duct work, but protects the duct work from any water damage. This insulation can withstand average flooding, and typical water vapor from flooding. While it costs more then traditional duct wrap, it will last through floods and typical vapor issues.
When insulating your home inland, we prefer to spray foam the walls in our retrofit division. Our team will install a 12 mil scrim reinforced vapor barrier, and spray foam your walls for a completely sealed crawl space. Once this is done and we also like to condition the space with a supply and return duct. This in the winter time, will drive heat up through your floors making your home a comfortable paradise all year round!!!