I think that it was canon that he liked to use significant kills to make his horcruxes with, but was it ever specifically stated that they had to be significant? Was there any kind of ritual for them? I couldn't find anything like that, but it is possible that my internet-foo has failed me and I don't have access to the later books right now. Anybody remember anything like that?

 

If I remember it correctly, there was no need for the kill to be special to make a horcrux, and I think JKR went on record as saying that Voldemort cast a spell on himself before each kill so that he could break off the piece of his soul from the crack the kill made.

 

Yes. I believe Rowling also said somewhere that, for instance, the one he used on the Diadem was just some random Albanian that was in the wrong place at the wrong time. So not all of his were made from a significant death.