Hi SoCal Dude,
I think your question will also be influenced by the brand of kits and decals too.
I keep my stash in a spare closet. I don't know if the temperature really affects them too much, but i suspect the biggest issue will always be humidity affecting the decals, discolouration of decals over time and, to a lesser extent, insects eating your decals/boxes (has happened to me).
Here in Queensland we get very high humidity in summer, however, i've had kits up to 12 years old and never had trouble with their decals. These kits were mainly Tamiya and Hasegawa - i'm not sure how a cheaper kit, such as Academy would fair.
I don't think warping of parts would ever be an issue, unless you kept it in direct sunlight and/or had a weight on it. I'm guessing they heat the plastic pretty high to mould it and doubt temperatures we human can tolerate in the shade would be sufficient to warp the plastic.
When you say you're keeping them in your garage - is that enclosed or partly open? I'd have thought if it's indoors, in the shade, the plastic will be fine.