I don't know which end is the "carto end," but dripping into the larger mouth end (right into the filler) is way easier than trying to drip into a little hole that's a tiny bit bigger than a needle.
Woops should of proof read I meant the atty/threaded end. Dripping onto the open filler is much easier but I thought you meant using the condom method on the mouth end of the carto's that h ave the sealed filler.