In regards to fly rods one of the biggest mistakes most anglers make is storing their rods in the wrong place or leaving their rods in a hot car. This is a big 'no-no' as the intense heat that will build up in these places is no good for fly rods. One of the first things that usually happens is the glue bond between the metal parts & the rod becomes compromised. Ever have a reel seat end cap or a set of metal ferrules come off the rod for no reason? You may, or may not notice it right away, either. It could happen months later, on a colder day when the failure takes place. The heat has softened the glue or epoxy & the bond was damaged.
Varnish is another rod element that can be damaged by exposure to excessive heat. What happens is the air gets so hot that it actually softens the varnish slightly. If the rod is being stored in it's bag, the surface texture of the material can be slightly imprinted (for lack of a better term) into the varnish. We see this a lot on the varnish over the thread wraps. The varnish is no longer smooth & clear as glass, but has little bumps & ridges in it from where the bag touched the varnish in the heat. In very extreme cases the varnish can even sag a little on the wraps.
Give some thought to where you store your fly rods. It might be easy & convenient to keep them in the rafters in the garage when not in use, or in a storage unit you rent near by, but all these places can get extremely hot. Also beware of keeping your fly rod in a hot, parked car all day long. Leaving a fly rod in a parked car in the summer sun is one of the most common reasons a rod suffers heat damage. It's easy to do, but resist the urge to keep the rod in the car all day long in the summer heat as the damage can happen pretty quickly.
So, please don't keep your fly rod in a hot car for extended periods of time. I've known anglers who have taken their rods to work with them in the morning, planing on a trip to the stream after work, only to find their ferrules coming off the rod, or other problems when they went to rig up. If you can, take your rod into work with you. If you work in a hot environment, like a factory or shop, etc, then find a safe, cool place to store your rod for the day. This is good practice for not only bamboo rods, but fiberglass & graphite rods as well.
No, you don't have to worry about your rod in a hot car for short periods of time - like a trip into the store or while you're getting a bite to eat in a restaurant. Most fly rods don't need to be kept in a special chamber - they're not priceless artifacts - but there is a limit to how much heat they can take. Just use a little thought & common sense & you'll be fine.