Inadequate rinsing can leave detergents in fabrics. I remember when TIDE XK was introduced with enzymes. People started getting rashes around waistlines from the enzymes left in the elastic starting to work on the proteins in their skin because one rinse was not enough. Now, with the use of non-phosphate builders in detergents in place of phosphate builders, not only can the hard water minerals, which are not handled as effectively by non phosphate builders in powder detergents, build up in the machines, but they can also build up in the fabrics because they do not rinse out as freely. I experienced this first hand back in the early 80s when ALL was about the only low sudsing detergent on the store shelves and my condo had the Westinghouse stacked washer and dryer. I discovered after I bought the ALL that it was a no-phosphate formula. I had to add Grand Union's phosphated water conditioner to every wash and to a deep rinse to keep the towels from feeling like wire brushes. You also want baby clothes and especially diapers rinsed thoroughly so that traces of detergent don't irritate a baby's delicate skin and so that in diapers, the detergent is not put back into solution when the diaper gets wet.
As for cold water pre rinsing of the diapers before washing, with today's lower water heater settings, you would want to do everything within reason to warm up the tub and the load before the hot fill so a warm pre rinse would help raise the temperature of the wash water because the warm tub and load would lower the temperature from the hot fill less than would a cold tub and load from a cold pre rinse. If the wash is started right after the pre rinse, you can also benefit from the hot water in the pipe drawn up to the machine as part of the fill for the warm pre rinse.
