When the voltage in Lithium batteries starts dropping there is a significant loss of performance which should be easily noticeable. From my experience really the only way to drain Lithiums below 3 volts is to run them until the car dies or gets very slow.
If you do decide to buy Lithiums I would suggest that you just be cautious with them for several packs and check the voltage with a volt meter often until you get a "feel" for when the pack is getting low. I would also suggest checking the voltage of your pack every time before you put it on your charger. If you do ever drop a cell below 3 volts monitor it very closely when its on the charger to make sure you haven't just started the ticker on a time bomb.
If you're looking for Lithium batteries you can often find Lithium cell phone batteries very cheaply on ebay. As an added benefit these batteries have a 3 volt shut-off circuit built in but many hobbiest remove that circuit to save weight.
One more note about the safety of Lithium batteries. As I just stated many cell phones use them. If they were inherently dangerous the cell phone companies couldn't afford the insurance it would take to continue selling them.