I'm pretty sure my 68V DC driver supply is also floating, and has been for the last 3 years or so. Why not? So, it bounces around a bit, but considering that you are generating 4A+ spikes of current up the power leads generating lots of potential EMI, what does a potential difference between the wires and some arbitrary ground point mean?

One good point about leaving it floating is that if there is an accidental short to earth of a motor lead at some time, you are less likely to make a driver go pop.

I have my "naive" hat on here but isn't there an argument that says that leaving it floating is less likely rather than more to inject random noise into the environment?