Sure, if it's got leaves of three let it be. But how about this:
While #2 son, Flamebrain, was home this weekend he spied this unusual specimen of poison ivy growing in the yard. His comment was something like, what is this five-leaved stuff that looks remarkably like poison ivy? Well, it was poison ivy as evidenced by leaves lower down on the plant.
Sneaky stuff, poison ivy. Flamebrain and Treebeard both have spectacular reactions to contact with Toxicodendron radicans, making them hyper observant and attuned to its presence. I'm not sure I would have noticed this particular variation.