the temper of the metal? An aluminum rim can take some heat, but it should be even, and get it too hot, and it will change the tempering of the aluminum. Heat is used to harden metals. used the wrong way it can soften them. either way an aluminum rim with uneven tempering would not be a good thing. I'm not sure how much heat you can safely apply to a rim before it messes wit the tempering of the metal, but being unsure I wouldn't do it.

A hair dryer should be fine though.