For the last couple decades most U.S. comics have actually been printed in Canada, so it would be the U.S. editions that get shipped over the border.
It should be noted that SOME (certainly not all) foreign comics have been printed in the US over the years, including some UK editions, some Mexican editions (also printed in Canada too I think) and many many others. Its just not consistent and not always easy to track where they were printed.
It should also be noted that for foreign editions, there's a decent chance the comic was not printed in the same country it was sold in, especially in Europe where countries are so close together and trade is so easy, but supplies and labor are so much cheaper in some countries. For example, it wouldn't surprise me at all if German comics were/are printed in Poland.
So if you want to start hunting for original printing country.... well good luck.
If I remember right a good amount of the European stuff was printed in Italy at one point. Spain too if I'm not mistaken. Italy may have a lock on the market now with Panini printing as much as they do.