I use Medical Marijuana to treat my seizure disorder. Should I stop use to get an accurate EEG test? Or continue use to see the effectiveness of it.

Here's a thought. Marijuana will not affect your EEG, but it is not a good treatment for epilepsy. So you should consider stopping it anyway.
Define accurate. There are some seizure related changes on EEG that are obvious on any study and some that will only appear during a seizure event. Some with a true seizure disorder have a normal EEG and some with an abnormal study never have seizures.If you have a baseline abnormal study, another on MJ might be interesting, but you should work out the specifics of its use or the value of another EEG with your doc.
Comments. Medical marijuana may be beneficial in certain rare and atypical epilepsies, almost all childhood variations. Likely has a very modest effect on seizure prevention in adults, and if you do have epilepsy, definitely use a standard anti-epileptic medication. Yes, get an EEG, but also see a neurologist and get advice about the need for future treatment.