How much is a transformer's secondary floating?
There will, in general, be a voltage on the output with respect to ground with a high value of R because of capacitive coupling and leakage between the windings.

Even a screened transformer (where the capacitive current is conducted out to earth) will have some voltage on each output with respect to ground. If the construction is symmetrical you might see half of the secondary voltage on each output terminal. Without the screen you might see that plus perhaps half the primary voltage. 

I don't think there is a safety advantage in a resistor (it could be worse if the resistor can conduct harmful current from the secondary voltage) but it could be useful in low level circuits to keep the secondary from waggling around at 120VAC or whatever if no other ground exists in the circuit.