I've often thought about this problem. Over a short period of my career, I was able to fly an assortment of aircraft much as I pleased. There were no passengers...paying ones that is.
My main concern was for the airflow into the engines, (Darts and Speys) but despite quite dramatic maneuvers, there seemed little change in the displayed temps. Huge amounts of energy could be dissipated by slipping, and when returning to normal line flying it was certainly a comfort to know that I could achieve a very steep glide should it become necessary in an emergency.
As for the load on the (T) tail. Well the aircraft was brick-built, so it's hard to compare to modern kit, but gentle entry into the maneuver was just a natural precaution, it was coming out of the slip that was difficult to do gently.
DC3s were an interesting aircraft to slip. Full rudder was an almost everyday event in bush-style flying. It was one @#$% difficult airplane to get out of the sky. Dirty as you could make it, and full rudder, and it still took something like (from distant memory) 5 mins to loose 9kfeet. An awful long time if you're on fire.