I am a girl in engineering, and know several other guys in engineering in fraternities and also some guys/girls that are greek that are hard-core science majors.
Many of them NEVER wear their letters to any class. MOST recommend to not wear letters or greek stuff the first 4-5 weeks of class.
Their assumption: professors will NOT take Greek students seriously. Even if you are in your upper-division classes, they will assume you will try to slide by, ask stupid questions, and be a nussiance to the class.
In my experience, many of my professors didn't know how to react to someone who is smart and greek (all of us!)... let alone a female in engineering
My thought was always that it would be very positive to see a Greek student with letters it difficult classes.