Okay, first thing's first:
Stop fucking fighting over this.

The safety word practice is actually a good idea. I know from when I was increasing my pain tolerance with burning and shit like that, I'd automatically start cursing and yelling stop and shit. My friend came up with the idea of saying "ENOUGH!" when I'm at mind's end. That way I could curse and yell "FUCKING STOP! STOP! STOP!!!" all I wanted but still get going. Now I'm much more pain-tolerant (used to be a bitch about it) and I agree with the safety word usage. It's much easier to get rid of the human impulse to keep going when someone's begging you for mercy when you don't use a word hard-wired in to your brain to mean NO MORE!...I suggest switching words once in awhile

The hole problem with saying "STOP" is that some kniky shit might mess with a guy or girl's head. They might know "STOP" means no more....but they'll keep going to go beyond the realms. But it makes it much more personal if an agreed upon word is chosen. Because then, someone's more likely to think "Okay, my love has had enough" Instead of "Stop? STOP? HELL NO MWUAHAHA!!!"

End rant