Here's a silly question, but as my great-great-grand uncle always used to say there are no silly questions, only silly great-great-grand nephews, so here goes:

You make a coin transport from one hand to the other. What do you show first--the appearance of the coin, the absence of the coin, or show simultaneously?

Gooooood question.

Usually I'd show the empty hand just before revealing the coin in the other hand: not even a full beat later, just by a whisker...

That's an interesting question. The most common situation would be a closed hand coins across. The gesture or toss causes the magic to happen,and perhaps a gesture of magically receiving the coin, or a clink to signify a magical moment. The reveal in most cases would be a simultaneous opening of both hands AFTER the magic has happened, as implied by your gesture. HOWEVER, Your words may dictate your action - The coin vanishes from HERE (Show the coin vanished), and winds up HERE (Show the coin in the other hand). No wrong or right, its what fits.

Another choice motivation may be the technical get-ready for the next magical travel. If, for example, the next passing involves a steal from heel clip, we may want to show first the hand of departure. Then as we show that, indeed, the coin has arrived, this supplies additional misdirection for the hand of departure to close and bring the next coin in heel clip position (with Michael Rubinstein's ring finger positioning against the edge of the coin for a later "kick").
The same logic may be considered if we're about to use a Han Ping Chien move over the spectator's hand.

There are many alternatives that rely on Rhythm more than anything else. Sometimes the action of opening and closing the hands can happen so close that it's practically something simultaneous, however, that kind of effect is better when followed by a series of similar effects (A flurry, as many people call it) Eric Jones shows this very well in the final phase of this fingertip coins across (The one where he uses first 3 coins, then 2 and then 1). I like to start with a very slow, effect, and then spice up the rhythm (People love to see some unexplainable, impossible and very fast magic when the moment is right) and finish with another deliberate, dramatically enhanced effect.

One of the things I've found after experimenting a lot is that no matter how cool fast is, the impact of very clear slow magic is always bigger. But you should vary the speed in order to decide how each effect will be perceived.

Tom said it best and to the point

"what comes next" is always a consideration in such decisions. Not only might the orchestration of the display of the hands lay groundwork for the next Move or Effect, but your actions may create a "Pattern of Performance" that will influence the spectator's Anticipation and acceptance of what is normal for the flow of the routine.

If you are performing a single, stand-alone effect, then methinks the "off hand" should always be displayed first to enhance Anticipation and remove any "other hand" suspicion. The memory of the event that you wish each spectator to leave with may be Surprise or cumulative astonishment. Whether they consider it magic is another matter.

I may be wrong, but thinking back to the different performances of the spongeball 10 count I've seen, I think everyone who I've seen do it (on DVD that is) shows the empty hand first by a little bit. But that's not quite the same effect I posed in the OP...

I think to make the question more interesting, let's set aside mechanical necessities--granted, sometimes you need to do it in a particular way, so the question is now, which way would you want to do it. I'm completely open to the reply that it doesn't matter one bit, and to stop wasting time, but I thought maybe the experience of the coin magicians here might have resulted in some definite feelings.

Did the audience expect the transit?

Did your character expect the transit?

If your wallet suddenly appeared in your closed left hand would you go reaching for where you keep your wallet with your right hand? Or would you respond to the appearance and then inspect the thing to be sure it's your wallet and intact?

Basics.

if doing a repeat-procedural routine, like Roth's, someone remarked that there could be a ten little Indians type mystery when you ask which of the coins it was that travelled. Acidently picking up an odd coin and having that travel does play well and permits getting set up for a c/s/b later on.

I like JT's remarks.

Did the audience expect the transit?

Did your character expect the transit?

If your wallet suddenly appeared in your closed left hand would you go reaching for where you keep your wallet with your right hand? Or would you respond to the appearance and then inspect the thing to be sure it's your wallet and intact?

Basics.

if doing a repeat-procedural routine, like Roth's, someone remarked that there could be a ten little Indians type mystery when you ask which of the coins it was that travelled. Acidently picking up an odd coin and having that travel does play well and permits getting set up for a c/s/b later on.