I actually refill my cartridge, mine doesnt leak unless I over fill it. The "white" cig models tend to leak, i'm using the "Dura C". I usually dont let my cartridge run dry (to preserve atomizer life) and just refill it now and then - 2 - 3 drops seem to do it until it gets moist enough.
It seems to be hit and miss as to when the cartridge is completely finished, from what ive read, when the juice doesnt soak in so well, the atomizer wick has pushed down the fibres or you start getting a bad taste.
I've heard that dripping on the atomizer is a bit harsh, in comparison to using the carts, not sure how true that is but i've been quite happy using the carts.