I think you just have to decide which outcome, you personally, would prefer. If you're really not bothered by the extra ?10 then don't ask for it! But playing through the scenarios - would you mention the discount beforehand? This could certainly put you in his good books but what if he returns, or becomes a regular? He might continue with the assumption that your rates can be rounded down.
Or if you're just waiting to see how much he'll give, my concern would be, regardless of whether you care for the extra ?10 or not, would you want the sort of client who thinks its alright to just pay less because the cost is an inconvienent figure??
But as the other ladies said - this is more a matter of personal preference and what you feel comfortable with

Elisabeth x