Think the point to note is that the main safety purpose of earthing metal cases is to prevent any internal fault from making the case live. So for the average end user of a toaster or whatever with a metal case, earthing that case is a safety improvement over not doing so.
However, on a test bench where exposed high voltages are present, especially when working on valve (vacuum tube) gear or SMPS, the presence of earthed objects on the test bench is a safety hazard. This is because you can't get a shock from a single connection. So, you inadvertently touch the anode pin of an EL34 with maybe up to 700v on it, and you might get a slight tingle if your shoes aren't all that good as insulators. However, go to adjust a control on your earthed scope, and you'll be paying Emily van Dort a visit as the earth completes the circuit.
Also, earth fault trips can only protect against current flowing from the mains live to earth. That is because they don't actually measure earth current, they compare the live and neutral currents and note any difference. If the HV came from DC PSU in the equipment, then it won't trip. Thus while an RCD/GFI on the bench is a good idea, don't assume it will save your life in all types of mishap.
Basically, when working on exposed circuitry you need to be constantly aware of the risk of a hand-to-hand shock, since this is the most dangerous. Best approach is never to use two hands at the same time. Especially, be careful to keep your non-dominant hand OFF any earthed metal. If you need to adjust test equipment whilst also holding a probe, do so by insulating knobs only without touching the panel.
The danger of not earthing test equipment such as scopes and sig gens is that an internal fault could make the case live. If you only ever work on low voltages this is the best arrangement. For high voltage work the best approach is to run the test equipment via an earth-free isolating transformer. So in fact you need two isolating transformers, one for test gear, and one for the equipment under test. Using the same one for both is not a good idea.
A cutoff button on the bench is also a good idea. Especially if someone else has to come to your aid, it means they don't have to stop and think about how to cut the power. or whether it is safe to touch you.
Oh, and beware of building site 'isolating transformers' which are actually center-tapped autotransformers. Correctly termed tool transformers, these are NOT for testbench use and would actually make things a good deal more dangerous than with no transformer.
Bottom line is that there is no absolutely safe way to work on exposed electricity. Avoiding accidents is a question of using your noggin and being 'ahead of' things that could go wrong. But, isn't that always the case anyway?