I fly a Rallye 100 into 320 M of grass, over trees from one end. In zero wind I "MUST" side-slip to get down in order to leave a reasonable margin to stop.
It goes back to the thread on "real pilots", For some reason young instructors in the main don't teach side-slipping, many new PPLs don't know about it nor when to use it.
If you must come in "high" over trees etc, or you "end up" too high on approach, you want to loose as much height as possible while travelling the least forward distance.
Therefore if you are still high while flying at minimum approach speed then you must "make" the aircraft fly less efficiently to increase the rate of "sink", side-slipping is the safest way to do this.
The C172 with 40 degree flaps should not be sideslipped with "full" flap. This is not because the aircraft will fall apart, but because the vertical fin could stall due to the disturbed airflow over the fin and rudder at a slow speed. As you already may have input full aileron, the aircraft could roll unexpectedly and perhaps through 90 degrees before you are able to recover. (not something you would want to happen close to the ground).
To side-slip without full flap in a 172 with 40 degree flap, will not give you any advantage over just using full flap and keep the speed at min approach speed.
Go out and learn to fly cross controls, find a good old "cub" pilot to go with you.
Tony