Unlike the evaporator, the heater core is a relative "piece of cake" to get to and replace when it fails. Which it will, eventually, but that might noit be for another 2-4 or 6 years. Or it might happen in 2 weeks. Since access to the heater core is relatively easy, unless you're getting the characteristic puddle under the floor mat, I'd wait.

If anything, you might want to replace all the dash light bulbs while the it's apart. They're cheap enough.

-Punx