If the fence is hot enough, they will honor it. I had the same concern until I did a few thing: 1. Purchased a very good charger (Speedrite 6 joules or better). 2. Improved the grounding system by driving three eight foot rods 15 feet apart. 3. In very dry conditions, I run a grounded poly wire a foot or so from the hot wire along the length of the fence.
Hope this helps.