Hunter is 3 and we catch him doing that too. Especially with raw hard foods like carrot or potato (thanks to MIL), as well as water. It took DH and I 2 hrs once to get him to swallow a bite of carrot that he was holding in his cheek. He wanted to spit it out near the end, but we felt that at this stage if we allow that, he'll keep doing it. So, we made him swallow it ... eventually. For the most part though, like Jessica said, we ignored it unless it was a dangerous food, like grapes etc.
Beth