This is a bit technical, but does a good of explaining theory......
http://www.kencove.com/fence/7_Volts+vs.+Joules_resource.php
The article references "weed burner" fencers, which likely could melt some of the new poly fences. Thankfully, most low impedance fencers sold today don't have that feature.
I run a 3 joule fencer on short lengths of poly tape fence, and have not had any issues related to too much power on it. If you use wires, there is no concern about high powered, hot fences.
In the end, goal should be to test somewhere between 5,000 and 7,000 volts on the fence. My preference is to "light em up". I don't get many customers coming back for seconds.