The only time I check ammonia levels is if I'm treating my tank for some kind of illness. Sometimes the meds can mess up the cycle. Also, if my fish start to act or look different and kindly sickly the first thing I check is ammonia and nitrites. Other than that I don't really worry with testing for ammonia or nitrites to often. I keep a pretty close check on nitrates though.
Oh, and if you add new fish you may want to test for ammonia as well.