a medical reason for a pre-shave prep of a "lather and rinse" with soap and water
A simple lather and rinse with regular soap serves as my face prep. Washing with soap reduces the number of bacteria on the skin, although it can't "sterilize" the skin. (reference below for a hand washing experiment) Theoretically, that should decrease the chance of redness and irritation caused by bacteria getting into small breaks in the skin. Lathering with shaving soap and shaving without first rinsing-off don't do the trick, because the bacteria are still in the lather until rinsed away. After the initial lather and rinse, the switch to shaving soap gets me the shave performance I want, on a cleaner face.
Shaving after showering would accomplish the same thing, I suppose.
I gave this advice to the concerned mother of a young person plagued with bumps and redness after DE shaving. A simple soap wash and rinse, followed by a standard DE shave did the trick.
I use an olive oil based soap with lavender essential oil which purportedly has some antibacterial effect. The soap and water prep seems to work whether or not I use lavender soap, which I choose for its fragrance.
Reference:
M. Burton, E. Cobb et al,
The Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands
Int J Environ Res Public Health. 2011 Jan; 8(1): 97–104.
Published online 2011 Jan 6. doi: 10.3390/ijerph8010097