I have attached a map of pickup and drop zones in the North Atlantic from a US perspective. You have to remember that if your Eastern fleet is dropping into Archangel or Karelia from Z04 then you can’t get to the Baltic in one move. You are stuck with going to Norway or WEU for a turn before moving to the Baltic.
The Baltic is generally preferable, though not always possible.
BTW - This is from Caspian Sub’s paper on US shipping (Paper #2). http://games.groups.yahoo.com/group/Caspian_Sub/