If you don't have to worry about permits or inspections, than you can build it however you want.
I would say it depends on the pitch of the roof, the strength of the ridge pole, and the strength of the rafters . The less pitch you have the more the rafters will want to push out.
Rafter ties would be a good idea, and will definately help alot.
I have built shed roofs (lean to's, porch roof, etc.) without doing a birds mouth cut on the rafters. Instead I ripped the header that the rafter sits on to the angle of the roof.
A birds mouth cut isn't that hard to do really, but it would be simpler for you to watch a youtube vid on it than for me or someone else to try to explain it on here. You need to know how to use a framing square.