Adam, in response to my steerer's seat pictures is quite right in saying there is a risk in sitting or standing within the arc of swing of the tiller. I was waiting for someone to make the comment because it had occurred to me too. However I will sometimes use the seat in that position for two reasons. The first is that I have a foot problem which has been causing me "toothache" in my heel all summer if I stand a lot, so I'm desperate to sit down when I'm steering for a long time.
The second reason is just one of risk assessement really. I reckon that on stretches of deep river or canal away from places where oiks dump trolleys etc in the water, the chances of getting knocked off the back by the rudder hitting a submerged object are pretty slim. Yes we do hit such objects, but not ever yet one which knocks the tiller through a full arc (or anything near). Just like every time I cross a rain soaked lock beam or step ashore onto a muddy slippy bank I have to take risks. If canals were new things to be built today, they'd never get past the Health & Safety lot.
Nevertheless I am not completely gung ho. I have been trying out the seat in postions forward of the tiller with some success in some conditions. I need more time to experiment really.