In fact, I never managed to actually get it to show any signs of low toner. I just got sick of taking the cartridge out and putting it back in to reset the "out of ink" setting.
Crooks!
a) When the cartridge has no toner, there is still a 'buffer' of toner that is in the developer roller that transfers the toner to the drum.
Now, it will last for quite a few copies, until it becomes completely depleted, and then developer itself starts transfering to the drum; Best case it causes premature wear, worst case it does serious damage and you end up with a costly repair bill.
b) When a laser detects low toner density, it ups the voltage bias to compensate, this too is bad for the drum, but more importantly, when you finally add that new toner cartridge, the printer will be suddenly 'overtoned' and has to get rid of the excess toner from the developer roll somehow. The way it does this is to print black copies (which get cleaned off the drum, straight into the waste toner container).
I'm not saying don't use the toner cartridge until it's completely empty, since sometimes the sensors do misdetect the toner level and report it empty when there is a small amount left - just don't over do it.
I'm sure the low ink warning is justified somewhere on the grounds of ensuring consistent quality, but as long as the prints come out good enough, print away.