I tuck in my shirt into shorts/tights when running.  Not always, but often, and often after I already started running.  I do it because it gets annoying and it feels like it impedes movement.

I perused the interwebz looking for a consensus on the matter (because I have nothing better to do with my time) and there is none:

• "I think a distinction has to be made here between racing and training. If you are talking racing, and hence dealing with a singlet, tucking is simply de rigeur. It's just the way you do it. It looks good, it feels good, it's clean and mean. Leaving your singlet untucked is a fashion statement someone like Garth Brooks might make, but it's not one you as a pistol-packing lizard want to commit. If you're not going to tuck, please also start wearing knee high white tube socks with your original Nike waffle trainers and wear a red white and blue head band. 

If you're talking training, it can go either way depending on the shirt. Sometimes if a shirt has a long tail, it's harder to tuck. But in general, tucking is cool and classy. Untucking is right up there in the cool department with leaving your last car parked permanently on the front lawn."

• "Depends on the shirt and the shorts...
  
  Longer shorts...untuck goes better....
  
  Shorter shorts..tucked in is better...
  
  Bigger belly....untucked is better...
  
  No Belly...tucked in is better....
  
  Short short and untucked...that looks bad..
  
  But,...again..we are guys..should we really care?"

• "Once I finally lost the puddin'-pop love-handles, every (sunny) day is a no-shirt day on the training run.
  
  For racing however, it is simply what one does. To paraphrase Johnny Cochran: If you don't tuck, then you must suck.
  
  On those cooler days when extra coverage is necessary, I tuck only a handful of shirt in to the wasteline at the small of my back, offering an elegant, flowing quality to my ensemble. It is really quite cosmopolitan."

• "To me it has to do with training or racing and weather.
  Training in good weather = my shirt out
  Training in cool weather = my shirt in
  Racing = always shirt in... Doesn’t USATF require shirts to start tucked in during official races?"

My take on the matter; do whatever the hell you want.  lol

On the second point - all the more reason for me to keep it untucked, don't want to look like I am trying to emulate an elite. It's like back in the day, if you showed up to a pickup basketball game wearing Air Jordans, you better have game. (I didn't, and I didn't.)

Word.  I never owned a pair of Jordans so there.  lol

Thank god for short racerbacks. They make life simple.

First of all, great pic!

Second, the only time I have ever thought about tuck/untuck is when it is cold outside and I decide to tuck in my shirt just to try to stay warmer.  If I get too hot during my run then I can untuck and let in some chilly air.  During the warmer months I'm always untucked and have never even noticed what other people are doing at races.