Isn't using a lower temp water more to do with not harming delicate fabrics or fading colours?
The hotter the water, the more germs, would be killed. 90 degrees after all is practically boiling your clothes.
The recommended washing temp on my sons cloth nappies is 60, so I guess that is adequate.
I wash whites in 60, sheets at 70, colours in cold.
All my baby's clothes I wash at 60, seems to work ok.
I don't think washing your whites in 90 degree water would turn them grey. I would have thought the opposite.