Re: Reloading in the garage??
No, it won't hurt a thing. May not be the most comfortable place to load, but you'll be okay. Cold ammo will perform a bit differently, but that's simply because the powder is cold when you fire it, not because it was cold when you loaded it.
Heat is another matter, when it comes to powder storage. Definitely want to keep it in a cool, dry storage area, such as inside an air conditioned house. In the rafters of an uncooled garage, or something of that sort, will cause it to degrade prematurely. And that WILL affect performance badly. Aside from that, you should be just fine.