About wing icing on top of the wings of a Boeing 737 NG I have to mention at least the following:
"...Thin hoarfrost is acceptable on the upper surface of the fusalage provided all vents and ports are clear. Thin hoarfrost is a uniform white deposit of fine crystalline texture, which usually occurs on exposed surfaces on a cold and cloudless night, and which is thin enough to distinguish surface features underneath, such as paint lines, markings or lettering..."
The text above is an exact copy taken from the supplementary procedures in the operations manual from Boeing. I would say in some cases it is possible to have a little bit of ice on top of the wings. I agree with the comments made earlier in this topic, I just wanted to let everybody know that there might be an exception to the case.
Always fly safe!