Ermm... I wouldn't say there is much difference from what I've seen. I wait several days before adding water, but not because of salt. The longer you let water sit after dechlorinating and mixing, the more undesireable trace elements in the water get neutralized. If you're using RO water, I don't think you need to wait a full day with a normal salt mix unless you are doing a large water change.
Maybe I'm misinformed on the RO process, though.