Isolation transformers need to have their output referenced to ground just the same way as any other transformer.
NEC says that if it CAN be grounded, it SHALL be grounded.
What doesn't need a ground connection is an "isolated power system" such as is found in a Hospital. But then you've got to have monitoring equipment to tell you when there's a fault in the secondary. There are also a few other very limited exceptions, but isolation transformers aren't among them.