Concerted erections are no problem but the spontaneous and accidental erections go the way of hair and having to bow to the toilet to pee in the morning. Sitting or standing work equally as well but I've never tried kneeling.

Whether you are a pop top or a hose, a strong stream means you are good to go.
Of course there are many types of penii; baby bird, Nazi helmet, opposite color, veiny, smooth, warped cue, reverse curve, too big to park, too short to knock on the cervix door and different growth ratios. Not that I am an expert. Now lets list the different kind of valleys. Genitals are generally over rated.