In my case, the genuine answer is that "it depends," on a multitude of factors. Generally, I'll piss in the cubicle if there is a crowd, but if the urinals are empty, I'll piss in the urinals.
Since I hang at around 7 soft, I'm quite noticeable and generally have guys clustering over to take a peek, which either annoys me if they're visually displeasing to me (hairy, unkempt, unfit), or turns me on if they're hot (fit and smooth). Usually has been the former, which is why I tend to use the cubicles, since although I love the attention, I don't like the attention of those that I would never consider fucking.
Which is why if someone who I find unattractive comes close to me to stare while I'm pissing, I'll usually stop mid-stream, cover up, and go to the cubicle to finish off, and leave the restroom as soon as possible. [Which is most of the time.]
If someone I find attractive stares, I'll usually side glance at him, smirk, stand back slightly and do a helicopter and let my cock hit my trouser fabric with a loud thump, before putting it back in. I'll then ignore the guy and go wash my hands, before turning around to stare him dead in his eyes for a moment. If there is no one else inside, I'll walk into a cubicle and leave the door open, while still staring him down with a smirk and see if he joins me in the cubicle to get his throat stretched. If there are others inside, I'll smirk and walk away, knowing that I'll most likely bump into him again in the near future, with him having that visual of my soft cock seared in his memory.