I've never sideslipped a 172 either.
But it strikes me that there are two possible reasons for such a prohibition. One is that it creates a handling problem, most likely longitudinal or directional instability (I saw both on a prototype once when sideslipping with full flaps, quite exciting). The other is that there is a combined rudder / aileron / flap (probably the last two) stress case which is considered hazardous to the airframe and therefore was prohibited.
Now if shortstripper has found no handling problem in sideslipping a 172....
G