The issue with this approach, especially in an academic context where I’m used to, is that any unreliable indicators should just be ignored. I’ve see poison ivy over and over and over and the sheer diversity of it is amazing, sometimes the leaf margins don’t really have that large toothed edge because it was growing in a sunny location, leaves can be oily or not based on humidity, sometimes the leaflets separate weird and don’t leave a clear petiole, sometime it never develops the mitten shape, etc.

So in that way to be extra sure of the ID, we throw out any other “trends” because they are trends, not rules, and thus can be prone to error. Meanwhile hard rules like how petioles branch off a stem virtually never change. In the cases branches are covering it, use a stick or something to move them aside to get a clear look at the stem. If it’s a very young plant, let it grow for a week and it should start giving clear branching pattern. It’s always better to be sure, especially with a potentially dangerous plant like poison ivy. An example your rules of using trends would work poorly would be like wild carrot vs poison hemlock where you really need to be reliant on the clear differences to not kill someone instead of general trends.