The reason why the manufacturer's have the tank not really show ink to be visible is if the sponge is too full and the ink can possibly leak into the printer which can cause major issues to the unit itself. You may be able to trick it for so long, but it's not recommended and keeps your printer safe.
If it gets close, I'd wait until it gives a notice that it needs replaced. Normally, it will just inform you it's getting close and to be prepared to get a new one so you're not waiting for it to come in and be down until it arrives.
Hope this helps.