When the original Canon Pixmas (iP3000, 4000, 5000, etc.) say Low, you can ignore that. When it stops and says one or more is OUT, then is the time to actually eyeball the tank(s). You can use it down to all that's left is in the sponge.
DO NOT let it go until you see print degradation. The ink also acts as coolant for the printhead - running nozzles dry can cause them to burn out.
.bh.