What I have always operated under is to classify to the lowest level you are likely to find plus 1/8". Where I am, finding anything larger than -20 is a pipe dream and most of it measures -100. So, I classify to 1/8" when panning. But when I am running the highbanker I don't classify what I put into it. If I happen to shovel up that 1 in a trillion 1/2" nugget, it should be in my nugget trap or in my mats. Even then, I like to run a detector over my large tailings because you never know when a miracle could happen.
When I am panning and I get down to the bottom of the pan, even if I don't see any colors I put everything that is left over into a bucket to go through it later at home when there is more time. It is good practice panning to have just the heavies and you would be surprised out how small the gold in the bottom of the pan can be that you will find at a later date.