The law says you absolutely are required to pull a permit for any change to a wiring system except very minor ones like changing a receptacle, GFCI, light fixture, or switch. This includes anytime you add wire such as extending an existing circuit. That said, it is rare to see an electrician actually pull a permit for very small jobs. However, a panel change is not a small job.
I would add this. It's not unusual for inspectors who are driving through a neighborhood to stop when they see two of more tradesman vans at a residence to see if a permit is displayed. If you are caught doing work without a permit you will receive a penalty which ranges from paying double for the permit, to being reported to the electrical board who can fine you and pull your licence.
You can also get caught by a home inspector or insurance inspector who looks up permits in the public records and finds none where pulled. This will cause the home owner concern and they might call up the county to ask questions. Questions you don't want them asking.
Don't be afraid of the inspectors. Most are very nice and will answer questions about a job before you start it so you are sure you are doing it right. You will get red tagged a few times. This is a learning process. After a while almost all your jobs will pass first time.
I personally rarely wait for inspectors. You don't have to be there, but probably will want to for the first few so you can get a feel for what the inspectors want. My contracts include language advising clients they are responsible for being present for the inspector. This lessens my costs and the price of the job. If the panel is outside and accessible (no locked gate or dogs) then nobody has to be there.
