I wonder what method results in better good will? I would guess not putting it on the invoice since putting it on the invoice a) moves the social aspect/norm to a market norm/aspect and b) shows you track time and are knowingly and willing to give some away for FREE (which I would assume might lead to a client abusing it later or questioning why or pointing out that you didn't charge for X or for that one time but now you are charging me this time or for similar thing Y and then having to explain yourself.