It's not a good idea to wait until the battery is empty to recharge it.

What I prefer to do is use a desktop battery charger, so when my battery runs low I just put in an already-charged battery. I don't like to spend time with my phone plugged in to a charger, it makes me less mobile. Total cost for charger plus spare batteries -- less than $20. This assumes you don't buy from Nokia, which apparently doesn't sell a desktop charger anyway.