The thing to bear in mind is that once established a raspberry bed may last ten years. So think of the future and position them as far as you are able to make keeping them weed free easy. Raspberries choked with weeds are very hard work. It is probably equally important to clear the patch of perennial weeds as best as you can before you start. I don't think the exact positioning is as important as decent access in order to weed, prune and harvest.