This is a subject which has been debated and studied to death, including in engineering forums like PM Engineer. I don't know if there is a "final answer" right now. IAPMO is also studying the issue with regard to code requirements.
Another issue under study is possibly reducing the size of the hot water supply to bathrooms. With everything low to very low flow, a lot of heat loss energy and water down the drain could be saved with smaller pipes, and possibly little or no effect on flow to the shower. Film at 11:00