Another vote for alcohol/water mix, I keep it in a spray bottle. Also use it for putting 3M paint protection film on cars, along with a bottle of baby shampoo/water. Since I have mistakenly grabbed the wrong bottle at times, I can tell you that the alcohol/water mix is indeed better for sharpening skis than the baby shampoo mix, by a long shot.
I have used alcohol to lubricate a tap when cutting threads in plexiglass, and it works well in that application (and evaporates so the holes don't stay wet or dirty).
If there is a downside to the alcohol/water mix for skis, it's that it can degrease and dry out the ptex bases. But generally we are waxing afterwards anyhow, so no big deal.
I don't like the idea of WD-40 near skis at all, since it could definitely block the wax from sticking to a base (it's often used to clean off crayon wax and goo, and does it by busting the adhesive properties). In liquid form, Alcohol/water doesn't agree with wax too well either, but at least it evaporates and won't be a problem later on. I think WD-40 will hang around for a while, until you attack it with a degreaser.