I've had to lift the scope ground in the past when working on TVs with a hot chassis. I did this knowing that I would be in for a jolt if I touched the scope case and Earth at the same time. I would always prefer to plug the TV into the isolation transformer and leave the scope grounded. Anytime your DUT has a polarized cord and feeds directly into a bridge rectifier, you're most likely dealing with a hot chassis and use caution to avoid tingles and arc welding.
Joe
And if I didn't ground the scope to the ckt, the line noise would usually swamp any signal I was trying to scope, PLUS I was forced to AC couple and that's not always a good thing.
