Just my two cents, but the only reason I plant on a slope is to try and hide the ginseng from the sun during the hot summer afternoons, the north and east facing hill sides will retain more moisture and give the plant its favorable habitat but as far as flat ground goes, if it is in enough shade with decent drainage, I would go for it. If there is any grass growing in some spots, its getting too much sun.
Down here in the south (I am in Southern Middle TN) and I think Benny is from Mississippi... we have to plant on north to north east facing slope or our seng just will not survive. It is just too hot and dry on the other hillsides.
Here where I live the North facing hillside will have ferns, seng, lots of companion plants growing nice and lush... but you cross the hollow to the south facing hillside and about the only thing that grows there is saw briars.
But the further North you go the less important that is. Some folks up in Main and NY areas say it grows just fine on South and West facing slopes up there.
Up north there in RI it may not make that much difference as long as you have good shade and soil.
One advantage that slope gets you is drainage. Seng likes cool moist soil, but not boggy wet soil.
I would say your success planting in flat areas will depend largely on how well your soil drains.
I have dug some nice healthy patches of wild ginseng in hollow bottoms here (On flats) but have noticed over the years that when that happens the soil is often rocky, sandy, very well drained.
Here in Ohio, I don't hesitate to plant southwest hillsides if they have the right shade and undergrowth. In fact, one of my best patches is in such a spot.
While the other's are giving their perspective (with which I'll not argue at all), my perspective is that the slope is normally for drainage purposes. I've grown ginseng in raised beds for years on flat ground. Remember, ginseng doesn't care whether there are certain trees -only that there is enough of some elements and not too much of others in the soil. Ginseng doesn't care if it is under trees at all -as long as there is enough shade to keep the surfaces of the plant cool. And I'd also say ginseng doesn't care if it is on a hillside or flat -as long as the soil is loose enough to drain well, yet organic enough to maintain necessary moisture.
My advice for new folks is always start slow and put out as many test plots as you can to test different areas you have to plant. Then, when time comes to plant the whole area, you know where the best area is each year.