Did some research via Boeing:
In a nutshell, dispersion is very quick. In one test, fuel was dumped on a bush fire type situation from 500 ft. No effect whatsoever. Most regs require a height of 5000 ft to make sure it doesn't reach the groung except in a highly dispersed state. In addition fuel is 'dumped' in small quantities in the flight refuelling process. The aircraft are generally close behind the tanker. Again no problem.
It is more of a piece of mind issue.