No one likes running out of ink unexpectedly. Luckily, printer manufacturers are making it easier to determine accurate low ink levels. Many modern printers will remind you when you're ink is getting low with others even giving you more specific input like the percentage of your remaining ink.
In some printers, a chip on the cartridge will tell your printer when it has low ink and stop functioning, forcing you to change the cartridge, even if the cartridge still has some usable ink left in it. This wastes usable ink and costs you more money in the long run. The manufacturers disclose the print yield of the cartridges, however, this may account for the amount of ink left over. Be aware of this feature when purchasing a new printer.
However, it is always safe to change the cartridge when your printer lets you know it's time to do so. Using a cartridge down to its last drop could potentially do irreversible damage to your printer.