As others have said, mowing up and down the slope is best, provided you have room to turnaround. If the slope is too steep you'll know it, because the wheels will lose traction and the tractor will slide down the slope.
I also use my tractor to mow the pond side of my dam, and it's fairly steep. I carefully back down the slope at a slight diagonal angle till the mower's crazy wheel hits the water, then I stop and drive back up the slope. I repeat this process, moving over slightly on the uphill portion of each pass as I progress along the length of the dam.
