In the case of MREs (and most other foods that are preserved by heat process canning), the heat/thaw cycle is not going to significantly alter the character of the contents, since the processing itself has already significantly broken down the constiuents.

One thing about canned foods. Freezing the contents can burst the seal. It isn't as bad with MRE packs because they have some expansion capacity, but the canned goods off the shelf at the grocery store don't have much give and won't take much internal pressure.

I prefer keeping dry goods in my car for emergency rats. They are less prone to damage and deterioration due to weather extremes. Of course, they don't take physical stress as well. My last cracker packet had been turned into cracker dust due to improper storage methods.