It matters if you bleach your white clothes.

Agreed, day to day you wouldnt notice the white changing but hold up a 6month old shirt next to a new one of the same type and its definately greyer.

Whites i always do on their own with a sprinkle of napisan with the washing powder. Courled and darks can be done together if the loads arent big enough to run two cycles. All loads i only ever wash cold, and half a scoop of powder max, the 'reccomended' amount is too much, especially if the load isnt completely full. I have never had an issue with smells, stains, dirty clothes with the reduced powder amount.

But i will never wash new dark clothes without giving them a couple rinses in a bucket of water first. Sometimes it takes ~8 buckets cycles of hand washing/rinsing to get the colour to stop running from jeans and dark items that first time.