"Cut deep and cut once". Multiple rounds of layoffs is going to seed paranoia as well as hurt morale, so it's often better to over-do-it a little than end up with some of your best employees quitting because they're tired of wondering if they're next.

I've seen both in action, and while the large layoff is really nasty, the company can immediately start to look forwards into rebuilding rather than sitting in the "was it enough, who's next?" purgatory.