It depends on how bad the rash is and what it's from. A mild rash will go away on its own, but some need the prednisone.
I get a tiny touch of poison ivy and it "spreads" throughout my body. To get rid of it I need prednisone. I used to not have any sensitivity to poison ivy, but once I got a scratch and it got under my skin. Now I get it BAD! I have to use plastic gloves when gardening now.