I've read anywhere from right at the edge of the counter to over the sink

Do what acm suggests. A bit of light will hit the mirror and reflect onto your face, which will help to alleviate nasty shadows.

Thanks for your advice....called our local lighting store, they suggest it be centered over the sink, definitely not behind you as you stand in front of mirror

I house sit for a family whose home has recessed lighting above the vanity in the bathroom I use. I HATE THOSE LIGHTS. Totally impossible to put makeup on or even get a take on how bad/good you look generally with those lights. The downlighting and shadows are terrible. Your situation may be different since you have the sconces providing some frontal light. Those lights are directly above the vanity and probably centered over the sinks. I don't really recall, even tho I've looked up at them, and cursed them a number of times!.

Another thought about the recessed lights, shadows aside, if you lean forward to get closer to the mirror to line your eyes or apply eye shadow, etc., you have effectively blocked the light from above.

Good luck! Getting everything right in a remodel is hard.

Definitely use dimmers and also be sure the warmth or coolness of the bulbs are consistent with the sconces.