Basic advice so far is OK. Beware of a few sets that use an autotransformer rather than a double wound transformer. These sets will still have a live (hot) chassis despite appearing to have isolation from a transformer.
The main use of an isolating transformer is to allow the safe conenction of earthed (grounded) testgear. if you're just working on a live (hot) chassis set without testgear make sure the chassis is conencted to neutral and use an RCD (GFI).
An RCD (GFI) adds no potection when using an isolating transformer.
Don't trust the switch on the set - always unplug if you want to be sure the set isn't live.
NB: I'm in the UK so using UK terminology with US equivalents in brackets. With 230V over here we can get proper shocks
