I read in some automotive magazine that the way a car looks from the rear is much more important than the way a car looks like from the front, because the rear of the car gets looked at longer while in traffic. You can follow cars for miles, looking at the back of them near constantly, and only glance in the rear-view mirror to see the front of car for fractions of a second.
Yeah, it's different for people, because you talk to the front of them, but by then, you already know what the front of their shirt says. To publicize and sell anything with T-shirts, it's probably better to put contact info on the back of the shirt. That way, when the line moves at conventions, you've got people gazing at the back of the T-shirts, and they can pull out their smartphones and check the site at their leisure.