I have a scene where I have a glass sliding door, which should be a rough greeny glass but not to rough, but what I noticed is that when I rendered the scene at 2000 samples the glass appeared to be less transparent then before, when I've viewed it at 10 samples.
If you look carefuly you will be able to see that the right picture is slightly more translucent then the left one, and where the change can't be easily spotted it can still make a difference.
If you zoom in you should be able see it more clearly