It depends on the mod. Mods that do step down can go much lower. The E-holly can do a .5 ohm build and it's a 20w mod. The Istick 20w, however, can only go down to 1 ohm. So, it depends on your device.
I would first determine the minimum wattage of the mod. If you don't know, post a pic and maybe we can figure it out.