Isolation transformer question

[yorgle]

I recently acquired a Tektronix 2225 scope and before I start learning how to measure with it, I figured I should first make sure I know how not to fry it (or myself).  I'm faced with the seemingly perennial question of do I need  an isolation transformer?  I've watched some Uncle Doug's excellent videos and wasted many hours reading online arguments, so I do understand that because the ground clip on the scope probe ultimately connects through the scope to ground, it can make big ZAP if it gets clipped to a hot or neutral point.  Therefore, the unit being tested must be isolated from the mains.  That makes sense to me if I were to go poking about about inside an old "hot chassis" amp.  But if the unit being tested is an amp with a power transformer, won't it already be sufficiently isolated (except for the primary side) as long as I use a 3-2 prong "cheater" plug on the power cord? 

[sluckey]

There is no danger when testing that has it's chassis bonded to earth with a green wire in a 3 prong power cord. Don't use a 3-2 prong cheater.