Yes, 'round it up' makes sense. It doesn't sound like a question, because it isn't a question form. Personally, as a native speaker, I'd feel very awkward saying 'Keep the change'. It's a cultural rather than linguistic issue, but I'd be embarrassed to say that - it strikes me as vaguely insulting to the taxi driver. When I take taxis, I might say, in a more casual manner, 'Call it eleven' or 'Make that eleven'.

Hi Crystal, That's alright and he certainly knew what you meant, but more common among English speakers would be: "Keep the change." (Ex: the fare is 10.50 and you give him 11) However, if you need change for a larger bill, (the fare is 10.50 and you give him a 20), you can say "11 is fine" or "(Could I get) 9 back" Hope this helps!