I did not really think about this, until I started riding a class 2A bike. I am skinny and bike is hell a lot heavier than a class 2b bike. I used to keep bike straight up while stationary, but now with a big bike, I prefer to tilt it to the left and have a constant pressure on my left foot. Bike will straighten itself when you move off.
For class 2b plank, I think its better to start the course with a bit more speed and use brake or half clutch to manage the speed later. Start too slow and the half clutch can easily put you off the plank, esp when all the bloody bikes in Ubi all behave differently.
Better be a bit fast and get through course and donate a few points than get immediate failure for dropping off the plank.
Another thing is that is you are nervous and your body is rigid, its even easier to lose balance, especially on test day. So better be a bit fast and let the bikes momentum keep you going in a straight line. Once you are stable halfway through, you also more confident and you can try and slow down a bit if you want.