I've taken to centering on the neck because sometimes the neck isn't centered on the shirt. If you center on the shirt and the neck is off-center, it will still look wrong.
Press to remove moisture first.Often you're told to press the shirt to remove moisture before you transfer - how would you keep the center fold crease if you did this?
You can buy cheap lasers from eBay and make lines on the platen to help with alignment. Several people have done it, try searching heat press ebay laser.Thanks for all the comments here, but I'm still unsure as to the best way to align a breast pocket type image when using heat transfers. My customer complained that 8 of the 36 shirts I sent him where not centered right.