I have a saddle valve, but I found that it compresses my cartos too much and then makes it harder for them to wick.
I switched to using a dremel with a cheap diamond wheel to slot the carto.
Works great. It's fast (I can slot a 5 pack in oh... maybe 5 minutes) and I can just hold the carto in my hand while I dremel it. My dremel has three speeds, and I use medium. That's fast enough to not take forever, and slow enough that I don't risk going too far and messing up the filler. Some people worry about the metal dust that it creates. I wipe mine off, but I'm not too concerned about any that might be left over.
I switched to using a dremel with a cheap diamond wheel to slot the carto.

Works great. It's fast (I can slot a 5 pack in oh... maybe 5 minutes) and I can just hold the carto in my hand while I dremel it. My dremel has three speeds, and I use medium. That's fast enough to not take forever, and slow enough that I don't risk going too far and messing up the filler. Some people worry about the metal dust that it creates. I wipe mine off, but I'm not too concerned about any that might be left over.