the bottom line is this:

-if you have a clean shit, that is to say it all comes out very fluently and without trouble, you will have a clean wipe, and standing wont spread the shit everywhere.

-when you have a not so clean shit, a diahrea shit if you will, there will be a lot of wiping. so when you stand, it closes the cheeks, and the shit will spread all across your ass crack. this will cause you to start wiping a much broader surface area, and consequently increasing the possibility of error, which we dont want.

conclusion: sitting and leaning >>> standing.