From just a few years cloning I found straight soft water & the right conditions best, because its most easy. If I did overdo e.g. willow shoot tip extracts, they sometimes did rot. Maybe bacteria...? Who knows, results matter in this case. So no more extra work nor money spending. I think it's also because rainwater is most easy to be taken in by very young initial roots.
The cuttlings are taken from a plant which is perhaps fed by an EC of 1.0-2.0. You can expect that it already has quite some nutrients in the leaves and internal fluids. These will function as a buffer as the plant overcomes the lack of water & the loss of internal pressure.
And because of the effect of the osmotic balance, the usage of rainwater or purewater is going to drive more water into the plant than if said water would hold any ppm/EC.
Actually the process of forming roots out of pure veg tissue is still a small mystery because the plant is able to revert some differentiated cells back to a "younger" form in order to have the root meristem cells back. The books state how the plant initiates this is still unkown... so perhaps a good idea to not bombard it overtly with hormones, electrolytes or acids.