I use an Aspire ET-S.
If it's kept filled and upright, it never leaks. If the fluid level gets low, small amounts of liquid can leak out of the bottom, out of the intake inlets of the coil. If the tank is set upside down, gravity will pull small amounts of liquid out of the wick and coil area, through the tube, and out the tip.
The same applies to all tanks, although bottom coils seem to be a little more vulnerable. But they perform better, and I would never consider switching due to the slightly high tendency to leak.