That's true. An old, now sadly departed, friend of mine came from Liverpool and grew up in the 50s and 60s. He and his mum supported Everton, his dad and his brother supported Liverpool. He's stood on the Kop wearing a blue and white rosette and apart from constant banter never felt threatened...
I voted 'no' which I admit is a bit hypocritical as I (probably along with most of you) have done it myself at away grounds.
However as has been mentioned there is a world of difference between sitting there quietly and not showing any visible support, and being an obvious interloper.