I'm admittedly a bit of a newbie and certainly a BI developer rather than a DBA, so I accept that I'm going to get a lot of the questions wrong. However, when an explanation starts with 'we'll assume' I feel a bit cheated. I know that assuming something is a certain way is a surefire way to waste a lot of time. I'm sure that everybody has had a conversation along the lines of:

You/them, 'have you checked..?',

You/them, 'it's definitely .....',

You/them, 'yeah, but have you checked it?

You/them, 'oh'

Don't get me wrong, I learn something every time, I just feel that answers that make assumptions can stack the deck against getting the answer right, particularly when the assumption can rule in\out a valid answer.