I can't tell you how many times I've filled a cup of brewed coffee too high, and the person asked me to dump some out because they need room for milk.
It's more efficient to ask them if they need room for milk before you pour the coffee.
But, when someone says that they want a cup of coffee, black... I automatically know to fill it up.
It's better to ask first.
Not everyone remembers to tell the server what they want. It seems that they think you're a mind reader. I hate it when someone orders a cup of coffee and then I pour it, and then they say "Oh, I meant to ask for DeCaf. It's good that they can't read my mind, because they'd really be shocked at what I'm thinking and wishing I could say - but I don't.
By the way, welcome to the Coffee Forums!
Rose