I do not carry to many so I put a slice in them. Some have one slice some have 4 then when I grab one in the dark I know by how many slices wich one I have.
I am assuming when you say you slice them that you are referring to making cuts in the call tape?...
Personally, if I have a specific call that I want to mark for whatever reason, I usually just mark the tape with a regular pen. As far as I know, I haven't poisoned myself yet, but a few people around here suspect the ink has affected me mentally to some degree...