The only reason any of my cats have been shaved (I've had four medium to long hairs over the past 9 years) was prior to a surgery. For the most part, my cats don't even get regular brushing--I've rarely had a problem with mats. When I do find a mat, I'll try to brush it out or just trim the mat off if it's too stubborn. My parents did have a Maine Coon mix that would get frequent mats--they had to give her the lion cut one summer because she had too many to be groomed out. If you have a cat who is prone to mats and has several already, then I would consider shaving. Otherwise, just start a brushing routine to prevent future mats.
If you need ideas and keeping your cat cool in the summer, check out this article:http://www.thecatsite.com/Care/9/Keeping-Cats-Cool.html