I've seen a number of pieces of mains powered test equipment which is not grounded including units with switching power supplies, and some of you may know that AC leakage current (typically around 0.2-0.5mA) in such units is at the front panel connectors which is caused by Y-class capacitors used for RFI/EMI suppression.
To avoid this kind of problem, I have a policy of using external power supplies with the mains ground tied to the secondary side ground (via an appropriately rated choke if necessary) and having the same for units with an internal power supply.