Many tire places are quite paranoid about this sort of thing, and for good reason.
Thanks to the massive lawsuits that have happened over tires failing (remember the Firestone debacle?), they are terrified of being targeted for another lawsuit.
Some states already have stupid laws, such as making it illegal to repair a tire if the damage is in the shoulder or sidewall of the tire. I've had tire companies turn me away because of a screw in the edge of the tread, but it was too close to the sidewall (though clearly still in the tread).
The spare on the

truck was manufactured in '94. A visual inspection showed no cracking, dry rot, or other forms of damage. The rubber was still pliable and it looked and reacted just like a new tire should (even had the little nubbies on it). Since it was older than 5 years, the tire shop was technically not supposed to reinstall it on the rim. Fortunately for me, the tech went ahead and did it as even he thought the tire was in perfect condition, and it was only the spare for her rig. (Plus she has Toyo M/T's on there....they'll never wear out.

)
This is a tire that's lived it's life in a moderate climate (Cali then Oregon), but has been exposed to the elements (hung in stock location).
As others have said, if the tire checks out, run it.