No boy in my class was ever slippered after coming out of the showers, and I never heard of this happening.
Boys were slippered quite regularly before going into the showers, but that was always over shorts for some misdemeanor through the p.e lesson.
I dont think it made much difference pain-wise, shorts then were pretty thin and offered very little protection at all.
I was slippered on numerous occasions 1,3 and 6 whacks and believe me my p.e master could slipper hard.
If he ordered a boy to wait for him after a lesson that meant a whacking was imminent, we could hear the howls of the unfortunate recipient as we were taking our shower.