Hmm..
By overseas I assume you mean having custom blanks made in other countries vs. sourcing ready-made blanks domestically, or having custom blanks made domestically.
Pros/cons
Domestic is more expensive than overseas.
However:
Overseas will require higher minimums than domestic.
Possible language/communication barriers with overseas.
Possible shipping delays with overseas.
Questionable labor practices overseas.
It's pretty much a question of:
How much money you will save
X
All the hassles you have to go through.
If you are doing such a high volume that you really are going to be saving a significant amount of money by going overseas, it makes sense. If you are just starting out, it's probably not the best choice.
I'm sure some other members will chime in.
Also, you should check out this thread:
how to outsource t-shirt production to china - any experiences?