Yes, wireless charging works even if you have placed a back cover on the device. Recently tested it with iPhone 8 plus and there was no issue in charging. No matter how thick a back case is there will be no issue in charging.
Well, I was testing it with the thin back case and later I tested it with thicker one too and didn’t found any sort of issue in charging of the phone. However, if you are having a metal case then it may create a problem. But if you are using a decent rubber, leather or a fiber case it won’t create a problem.