There is no excuse for poor butter/margarine etiquette. DO NOT use the knife you just spread jam or peanut butter with to then spread butter, just use a new knife you're getting the butter dirty.
I'm tired of opening my tub of spreadable butter to see it covered in a thin layer of jam, I don't want that crap on my toast or any other food that requires butter. Just use a new knife, we have a dozen of them that never seem to get any use!