3 y.O has grand mal seizure once monthly since sept. 2012 avg. Total length 10 minutes & at 3 minutes I admin. Rectal diazepam. Should I medicate daily?

Not unless instructe. Your child needs to be assessed by a neurologist. Never use any seizure medication in a way that was not prescribed by someone who specializes in seizure disorders. He may in fact need a daily medication, but which one and what dose varies by seizure type and cause, and you can make some seizures worse with the wrong medication.