Grounded shield is more effective against EMI (skin effect). At higher frequencies it is better to ground shield at both ends. USB power is dirty to start with, with or without a shield, and should not be used. EMI induced current in the shield has to return to ground or shielding will be compromised. There might be some noise induced into twisted pair carrying signal but most of it will be rejected being common mode. It doesn’t really matter, as long as noise is not big enough to flip bits, since asynchronous USB transfer is just a data without timing and therefore without jitter.