I'm not surprised that prednisone, a powerful anti-inflammatory, helps your Piriformis Syndrome.
I have also learned, through hard experience, that the stretching is a "forever" thing. If I get lazy about it, back comes the pain. I do a complete stretching routine before getting out of bed in the morning, and more after I shower.
Piriformis pain can also be related to hip issues and arthritis in the back.
Sue