Most likely answer, there is no definitive answer unless you wind test every single different scenario. And also, for the considerably largest part of your riding you are facing the wind at yaw angles. And those are for the better part larger yaw angles than you think due to the movement of the bike in relation to actual wind angle.
I was thinking along the same lines myself, although I do suspect there is a position that has a good chance of being near-optimal for the majority of set-ups. I guess the thing to do would be to look at the set-ups that the more "marginal gains" orientated pro teams adopt. I wouldn't put it past some of them to have tested this sort of thing in the wind tunnel..
Seriously? As I sit here sipping a glass of wine, I almost spit it up. If you can tell a difference in aero between a Garmin mounted in line with the stem (looks the best and easy to read as you mentioned) and in its most aero position (straight ahead into the wind) then bully for you. I'm sure you'll get some more useful answers than this. But i don't think you'll notice a difference either way.
Well, I hope I wasn't responsible for the loss of any good wine - as it happens was sipping a nice Pinot Noir myself when I started the topic.
I agree that any gains are highly unlikely to be noticeable, but I think it's reasonable to assume they might be just as significant as a whole bunch of things that some people (or marketing departments at least) want us to take seriously, such as integrated front brakes or cable routing. Unlike those things however this is free and can be tweaked in 30 seconds. So why would you not
want to do it if there was even a slight chance of an advantage?
What prompted this was reading a feature on the new version of the Boardman AiR aero road bike - Chris Boardman said that one of the most significant aero improvements in this version came from altering the cable routing at the front. The fitting instructions for Campagnolo ergo levers make a point of warning you to angle them precisely forwards for aero reasons. People pay good money for supposedly aero handlebars with aerofoil profiling on the top, etc...