IMO the real answer is it doesn't matter either way. Let them finish the word for you. I do the same thing sometimes with fluent speakers too I think it's more important to be 'in' the conversation with the other person. Granted if they get the word wrong it can be annoying, but if they're right, then the message has been successfully conveyed. Don't lose sight of that -conveying your message- and get hung up on individual blocks.