Looking at some statistics this is how the average household water usage is distributed:
It would be nice to supply toilets with RO waste water since contaminants would be of least concern. Perhaps one could again use an accumulator and a check valve to send RO waste to cold water supply but only in bathroom(s).
Laundry is another good use but perhaps in a lower concentration to prevent water stains. I'm not sure how much of an issue this is so perhaps people who have used waste RO water in laundry could share there experiences.