Nah, I don't like that. As far as Okay/Not Okay thresholds go, 0 is pretty much the best and most obvious value.
That said, I think dead at -10 is a bad idea, too.
Here's what I would have done: when a character reaches 0 hit points, he must immediately make a Fort save (DC 10 + damage done in that last attack) or die. If he succeeds, he remains at 0 hit points and is incapacitated. Thereafter, the character must periodically make Fort saves to remain alive. Any further damage suffered by the character requires a new save (but leaves the character at 0 hit points). A coup de grace is automatic death.
This would get rid of the problem that -10 is basically a trivial buffer at high level. It would mean that a character who is dropped to the equivalent of -9 might not bleed out before the Cleric has any chance to get to him. Conversely, it also means that the Cleric can't just leave a colleague who dropped to -1 for a few rounds because he has more pressing matters to attend to - his friend might die at any time.