Here's a link with some information. http://www.cpsc.gov/CPSCPUB/PUBS/463.html

As with all heaters, there is some risk. That risk is there for the under the tank heaters too. I wouldn't plan on buying the cheapest model out there, but look for one that meets federal regs. I know I've had good success with my electric space heater when I used to have to use one. The room they're in now doesn't require it now. I usually put extra smoke detectors in that room, and you can use a thermostat to help control the heat output of the heater. Otherwise most of my knowledge is very limited. Hopefully that helps some.