Most implements are designed to run at 540 rpm and do well at that speed. If you have a 1000 rpm PTO you can use it, but slow down the tractor RPM to account for the difference.
I assume you have a 2 or 3 speed PTO. When the snow is light it is nice to be able to slow down the engine and still get the same throw of the snow. When the snow is heavier, I'd shift back to 540, and speed up the engine again.