Great topic!
I'm no expert on the fishing side of this, but I would guess that if you are using a strike indicator to regulate the depth of the nymph like Jude, the issue becomes less important as your depth lessens and you approach the surface. As you approach the bottom, the currents become more confused from reaction to the bottom. Wouldn't the size of your indicator(drag) come into play as well?

I find myself nymphing on a downstream swing often, so I'm able to feel the take pretty well, but I haven't yet developed the feel for nymphing upstream without an indicator.

To get a feel for all the currents in a river, a great way is to actually swim in one...preferably one with lots of eddies and even some mild rapids - swim on the surface and then swim the same line subsurface, go into and out of eddies, feel the down pressure on a swirly eddie line... You'd really be amazed at how powerful some of the sub-surface currents can be in even a "mild" river.

Charlie