Hi', Phil. For me, there is no single right way to present a dry fly. Upstream gives us the advantage of a blind-side approach, and in running water, a bit of turbulence to mask it. Also, the most likely means of obtaining an easy, drag-free drift -- not on tails of flats and glides, tho -- and good hooking on the lift.
Across the steam, so long as the pace of the water covered is even over its width, gives us the opportunity to present the fish with a fly and very little or no leader to be seen, plus the opportunity to set the hook in the scissors.
Downstream gives the best opportunity to hide the leader, very often, the chance to avoid drag, but hooking is more tricky, and we have to learn to fish parachute presentation and slack line feeding, plus unobtrusive removal of the leader after each unsuccessful drift -- but you know all the above already!!
The main advantage of downstream presentation of wet fly is probably that it gives the fly more life, lively movement, than the upstream, tumble back down the water, dead drift, which is dependent upon the tried and tested simple spider patterns, the sparse and light, wispy hackles of which take on life and attraction even in light currents, without fighting the flow.
No method is best all round, each is best in a given situation. All have to be learned to be a complete angler; and there aren't a lot around.


Terry.
And I ain't one!!
It's always horses for courses to get the best results --- utility and versatility --- that is the combination.