Clean Your Palette!
It is best to clean your palette when you're done painting for the day.
If you've finished your painting, there is no reason to save the slop on your palette.
It is best to start with a clean fresh palette every time you sit down to create a new painting.
Taking the time to clean your palette at the start of a new painting, may spoil your enthusiasm to paint. You may end up loosing your muse if you have to clean up the leftover slop.
When you're done painting, clean your palette!