We have Mastersiana, Dyeriana, Mixta, Morganiana-all cultivars propagated by cuttings going back to the late 1800's and early 1900's and they perform beautifully.
Another important consideration when taking cuttings is to use sterilized tools, otherwise you could be spreading viruses to your cuttings that will then be carried by the plant and spread when cuttings are taken again. Not all viruses kill, some will make your plant "sick", slowing growth rate and weakening resistence to other diseases such as fungus. Imagine how many viruses can infect a desirable cultivar as cuttings are made over a period of decades without taking the proper precautions.
We use single edged razor blades, one per plant.
Josh, the Effulgent Koto is a grex name, so the "male" parent of the cross is most likely from another grex-probably using different clones of mirabilis and thorelii. The Japanese custom is to name the grex: example-thorelii x maxima is N. Rokko and N. Balmy Koto, and a few others, but using different clones for parents.
Trent