Kim I do know a lady up here that does white wash the bottom of her hybrid teas up to say three inches. She is just concerned with the bottom portion above the mulch line. I don’t bother myself because mine are almost all own root so if the die to the ground they usually come back from the roots. Native roses to the area never have this problem. Thinking about it many of the natives have reddish stems in winter and maybe red is the wavelength that causes sunburn. But that is just a guess. It does help a lot with fruit trees up here. Certain types will all be understocks in a few years if you don’t.