When you're venting, you're working off the roof ladder. And you'll want to stay out of the smoke/fire as much as possible.
When you make your first cut (away), then you can step back and stay out of any smoke that may start pushing out.
Next, you'll make the top cut, this again is so you can work under the smoke that WILL start pushing out of that cut.
Next, the bottom cut. This is because you want the closest cut to you last. If closest is not last, then you'll be working over top the smoke and fire that's blowing out. If you leave the bottom or top cut to the last, that means you already have 3 sides cut, and that piece of roof is VERY unstable. You definitely don't want to be reaching over that hole trying to reach the far corner with the saw, and risk a chance of losing your balance and falling on top of it.
Also, when you're cutting a vent like that, the chances are good that you'll cut through a truss. This in-turn weakens the roof. You'll want to be on that roof ladder as long as possible, as it will help distribute your weight.