No. It's not necessary. The reason everyone suggests using soap is that soap is a "sticker" and supposedly keeps the spider mites from coming back. But if you are washing your roses regularly with just water, you don't need a sticker because you have broken the breeding cycle. You will never have a serious infestation.
Actually, if you are running short of time, you don't have to be very careful to wash under the leaves, because just wetting down the plant changes the moisture level within the plant. Spider mites like dry warm weather, not moist warm weather. Just make sure you find time to hit the under sides of the leaves next time you wash the roses.
Smiles,
Lyn
