Give it as much distance as you need to get the wheel out and avoid ripping the tire on a trapped rock. Otherwise, the fender and the wheel both have an arc. There's a point of distance where the two arcs appear complimentary to your eye. That part is a ymmv thing.
Something that always looks 'funny' to me is a fender arc that suggests the wheel should have been taller or shorter. (or wheel arc that suggests the fender was for a taller/sorter wheel).