In my opinion, substrate heating is not a requirement for a healthy, long-term tank. In the past, I've used Dupla's and Amano's heating systems and I live in Miami where the A/C is on 95% of the time.
The point os substrate heating (that is debatable) is that it creats a slow current between the substrate solution and the water column. This brings freshwater to the susbtrate solution thereby preventing anaerobic conditions that may impede root growth. It is also thought to prevent compaction of the substrate.
People have considered whether the slow current is actually created by looking at the fluid dynamics associated with heated cables. I beleive the result was that it was questionable whether the cables did what was thought they did. There had to be a sufficient temperature differential between the substrate solution and the water column to cause some sort of water movement.
Take into consideration that the same water exchange is done by the plants. Plants transport water and O2 from the water column and release it into the substrate solution via their roots. This way plants make nutrients available by creating a anoxic (low O2) environment that ensure most nutrients are in a soluable state.
Compaction can be avoided by using a proper substrate base.