it is not only a maya 5.0 issue as i have that with maya 4.5 too - even when i dont load up mr every time i start maya. it is no use to close the window - as sometimes something does write to it - and it opens back up. the easiest way to get rid of it is to minimize it. then you just pretent it does not exist. whenever you render detailed results go to the output window (if you enabled that behaviour - not sure if it is enabled by default).

hope that helps