"Boy, if the manufacturer don't recomend doin' something,dont go stikin' yer dick in there!! "
I agree j_t, with all of your comments; proceeding with great circumspection in light of potentially significant hidden and/or known design flaws, cumulative airframe stresses and faulty maintenance practices, contributes greatly to a pilot’s potential seniority. Re Homer Simpson’s comment quoted above, however, most manufacturers do in fact recommend slipping the aircraft, albeit in the context of a crosswind landing. The Boeing bulletin kindly linked by dvt indicates that this maneuver may be safely accomplished throughout the flight envelope.
In my view, therefore, if dvt wants to side-forward slip his 737 till the cows come home, I see no safety-related reason not to. That the passengers might view things from a different angle is a horse, of course, of an entirely different color.