I don't check my levels, I also have well water....although having well water doesn't mean the ammonia and nitrate levels are guaranteed to be ok though does it? I really have no clue.
I dont normaly unless there is something bad going on. Then I test them and it always turns out they are fine. I have never had problems with cycling. I do my regualr water changes and dont worry about it.
overstocking can definitely creep up on you if you don't do regular WC. Another reason for testing is if you have CO2 injection. This can drastically affect the PH (and oxygen levels too of course). A decent master test kit is cheaper than restocking a whole tank, so IMO why chance not testing at least monthly?