For a trainer plane I don't think it really matters which is up and which is down, but for anything else you want your "up" (aerobatics people refer to it more as "top" for 'top of plane') line toward the TE, and your elevator control-horn on the bottom - so you've got max line tension for "top-side" maneuvers.

Excellent points made already about positioning of the belcrank - slightly behind the CG is best. Your tip-line-guides definitely want to be a bit behind the CG, just a bit more than the belcrank to allow for line bow in the wind while the plane is flying. Adjustable lead-out guides are great too. That way you can adjust just exactly where the lines come out of the wing after a couple flights.

The #1 rule in CL flying is, and always will be, KEEP YOUR LINE TENSION POSITIVE !!!!

Dave