Also, in a company that's growing 30% every year, hiring "10% too many" people at any given moment is an extra 4 months' of pay for 9.1% of your payroll, or about a 3% mistake in a 30% growth environment.

In a company that's growing slowly or not at all, hiring "10% too many" people is a much larger error, possibly even a disaster as it could be a full year or more worth of extra hiring in a slow growth company.