i think copper is best at level 16 and tin at level 35 from what i remember from direwolf's videos
my turtle "Copperdog" found quite a bit of it in the first few levels while excavating a chunk so all is good
I was just at a standstill because I needed copper for pretty much everything I wanted to make and didn't feel like strip mining near the surface
while that was going on I put up a large monitor (that actually needs to be a bit larger) that shows the chat from the Mindcrack server for stuffs and giggles

looks like avidya has a prank waiting for him :B