The more critical wing shapes we use now are far less tolerant of ice than the barn doors of old. The accident statistics show that modern practices of de-icing and being incredibly cautious of any ice, is the correct one.
I'm sure you cn "get away with" an awful lot, but why take the risk and where do you draw the line?
Ice under the wing has a smaller effect on the aerodynamics than on the top surface and if your Op's manual says it's OK and you are happy, then off you go, but when there is ice underneath, there's ice on top, so if the top needs to be deiced anyway, why not get the whole lot done?