(Seemed more appropriate for here than GQ …)
:: “GATTACA” MOVIE SPOILER COMING UP (as if you’d care at this point) ::
In the movie, Ethan Hawke’s character is discovered to be non-genetically altered by the guy who administered physical tests to him, because he held his penis with the wrong hand while peeing.
The test guy pointed out that you hold your johnson while doing Number One with whatever is your dominant hand.
Is this universally true? It is in my case, so it has me wondering.
I use both, myself.
Yes, it takes all the muscle power I can muster just to keep the mighty beast under some semblance of control. 
True for me.
It’s true for me. I use my dominant left hand.
Don’t you guys really sing a song … right hand… left hand… here we go!
Um, I’m right handed and use my left hand to ah, direct the spray. Does this mean I’m a mutant or something - or did I win the big prize?
Never heard that song, but as far as the OP goes, I’m true to that, too.
I’m mostly ambidexterous, with my right being dominant.
But I hold with the left.
Right, my dominent hand.
My plumber’s helper is my right hand, but my erection handler is my left.
True here, dominant right hand.
I’m right handed, but I use both when peeing (when the crane’s in the shop, that is). Interestingly, despite my handedness, I can only brush my teeth with my left hand. If I try to do it with my right hand, it’s all awkward.
I use either hand, but I’m always conscious that more than three shakes constitutes a wank.
True for me, right hand dominant.
I’m right-handed, but I use my left hand to aim and fire at the toidy.
(Yes, “toidy” is the official word.)
Generally the right hand (dominant) gets the job. Every now and again I let the understudy stand in. It’s probably the only thing I’m ambidextrous at.
I use my right hand when peeing, but I am an ambidextrous masturbator.
That should be a university degree. “Matthew McLauchlin, Ambidextrous Masturbator.”
Apparantly I’m a mutant as well, but enough allready about my large penis.
I’m right handed but my left does the aiming, go figure.
Righthanded in script and urine.
However, when I kick a football, I kick a lot better with my left foot then with my right - but I can do both. Not sure what the penis angle is in that last bit, but there you go.