Serial numbers should go on front. The front is the focus of the card. The back is usually filler. Also, please stop putting serials on non auto'd, non patched inserts out of /999 that no one gives a damn about.
P.S. If anyone from Panini happens to read this for some strange reason, please send me free letterman cards. I want to spell out, "I bought Panini and all I got was recycled cards from the last eight sets." That's a lot of letters to pull myself.