It seems to me that there are 2 reasons for using a preshave: softening, and lubrication.I'm not sure where to take this, apart from trying some pre-shaves after a face wash/oil removal, but I'm curious about others' thoughts, experiences and processes.
For the first, if you don’t shower before shaving, a preshave to soften your whiskers is probably a good idea. If you do shower, or spend a couple of minutes washing your face or applying a wet washcloth, it’s probably not necessary.
As for the second reason, I would think that if you need a lubricant (preshave) between your skin and your lubricant (lather), then there’s probably something wrong with the latter lubricant.