I say this because I plan on keeping the truck for a while and am approaching 70k. My thought is that if I reach 90k and haven't blown a head gasket I may just do them as preventive maintenance given the stories I've heard. Does this seem reasonable or overzealous?

 

Let them go as long as you can, that way if you are starting to get any valve guid or seal issue you can deal with them at the same time.
In the mean time, keep fresh Dexcool in the system and consider using a bottle of either Water wetter or Purple Ice for a little added protection.
Shout if you want my 60K service list if you want some direction.

 

Overzelous.
Change the coolant every 3 years or 45,000 miles like you are supposed to and you shouldnt have a problem.
I recommend replacing it every 2 years no matter what.
I advise against DexCool, but you can use it if you want, no matter what coolant you use just change it every 2 years.
Coolant is alot cheaper than headgaskets.